I am going to go straight to the technical stuff. I am going to be general, since we are talking about a pretty wide selection of applications. However, we would supply a transmission with the parts to bolt it to your engine. in 4WD vehicles, we would supply the trans so that it will bolt to your transfer case. Lots of variables, but in essence, we will put together a package that will bolt together. 

The fine print: As for "is it 100% Bolt-in", yes, all the components should bolt together fine.  However, I would never consider or assume any installation of a transmission that did not come in the vehicle was going to be 100% bolt in:   1. Might work out that way. 2. Most cases have a few issues you will need to figure out on the fly. 3. Occasionally there is some issue we did not anticipate that turns out to be a hassle. That is rare. In any event, our customer service is always available to help you resolve whatever issues may arise in your conversion. Our Tech Support is generally quite a bit more knowledgeable than the average Joe you get on the line!

The main issues you will run into:

Remove the old transmission from the vehicle

Check to see if the torque converter will bolt up to your flexplate. In most of our conversion applications, the factory flexplate will fit.  Occasionally, the only difference is the torque converter bolt pattern.  it is very simple to re-drill the converter mounting holes, and this is cheaper than replacing the flexplate. In packages from High Impact, we work most of these details out for you before hand. If you are trying to develop your own package, more info here.

Bolt the trans to the engine block and the converter to the flexplate.

Crossmember: These trannies are longer than their 3 speed counterparts. In short vehicles we will need to consider sufficient room to fit. Most vehicles have a support at the rear of the trans, supported by a frame crossmember. This will need to move back a few inches. With the trans bolted in and a new mount on the rear of the new trans, it is easy to see exactly how far. Often it can simply be unbolted, slid back, a couple new hole drilled, and presto. bolt it in in its new position. In some cases you may need to cut out the old one and weld up  new one out of tubing. On many jeeps, the rear of the trans is supported by a skid plate, and you can simply drill new holes in the skid plate.

Driveshaft(s): Driveshaft modifications are to be expected: the rear shaft It will need to be shortened. This is usually done by taking your old driveshaft to a local driveshaft shop and having them shorten and rebalance it. They usually charge around $100 for this service. In 4WD vehicles, the front shaft will also need to be lengthened.

Shifter: 

Modification of Factory shifter: In most cases it is possible to make a factory auto trans shifter work in a conversion application. Where the conversion is going from a three speed trans to a 4 speed (Overdrive) trans, it is usually possible to make the stock column shifter access all gears. The arm that attaches to the selector shaft on the trans can be modified. Its length (distance from selector shaft to linkage attachment) is altered such that by moving the shifter from "park" to "1st"  will move the trans selector shaft all the way from full Park position all the way into 1st gear position. P & 1st gear shifter positions will end up right on. ,R, N & 2nd gear positions will end up a bit off, but readable. "O/D" will be to the left of the old "D" position, while the new "D" or "3rd" position will be to the right of the "factory "ED" marking on the shifter. In some cases it is also possible to modify factory consol type shifters to work in a similar fashion. The key is a custom selector shaft lever, and in the case of cable actuated shifters, mounting the cable to something at the transmission end.

Aftermarket shifters are available to precisely shift almost all automatic transmissions. Lokar and Genie make some excellent shifters. We consider these the best for a road driven vehicle. Art Carr gate type shifters tend to be preferred by some in competition.  B&M make a number of cable operated shifters for a variety of applications. Mostly, these seem like a decent product, except we have had serious problems with the plastic cable melting when even within several inches of exhaust. At the time, we called B&M about this issue: They did not have a higher grade steel or stainless steel  replacement available, and were un-helpful. For this reason, we do not recommend these shifters unless you are sure the cable will go nowhere near any exhaust piping. 

Preferred models of these shifters are available from High Impact, and are optionally included in High impact conversion kits.

Tranny Cooler

In my experience, the factory tranny cooler is inadequate for any use of other vehicle other than what an average consumer of that vehicle would be expected to do. If you like to push the vehicle, add a temp gauge. And make sure you have enough cooler to keep it cool. Article: Making your Automatic Trans Live

4WD apps: Mating to the new transmission to the transfer case

If we are supplying the transmission, we will supply the transmission with the correct adapter installed. We can supply any additional parts needed to mate to your transfer case as a part of your package. In some potential conversions, the adapters are not available, and the transfer case will need to be changed as well. These are all factors that will be known up front, when we are quoting out a package for you: It will be a part of your quote if it is needed.

Bolting the Trans to the engine: Torque Converter Clearance

This is a caution for when you are installing ANY auto trans. First, slide the torque converter into the front of the trans. There are two engagement points, one is the splined shaft that transmits power out of the torque converter. There is also an outer sleeve with notches in it that must engage the pump. Slide it on, and rotate it a bit to make sure the sleeve has fully engaged into the pump. Now put the trans up against the engine and bolt the bellhousing up to the block (or adapter, etc). If it will not go all the way against the engine without the converter jamming against the flexplate, do NOT force it by tightening the bolts. Once the trans is bolted to the block: Verify the converter is not jammed in there. You should be able to rotate the converter by hand, and it should be able to slide in and out against the flexplate. Minimum about a 16th of an inch. If it needs to slide more than 3/16 or 1/4 an inch to touch the flexplate, make sure it is still engaging the pump.

1. If the sleeve is not engaged in the pump when you try to install the trans, it will jam against the pump hub when you tighten the mounting bolts to the engine, and will likely damage the pump

2. If the TC is not engaging the pump, the trans will not be lubricated (which will damage parts) when you start the engine, and none of its hydraulic functions will work (will not go into gear, etc).

I make mention of these points, not because it is complicated, or even any different than with a stock application, but because it is a common area of failure in installing automatic transmissions by persons not experienced at this. Simple, basic, but you DO want to know about this.

4WD apps: Front Driveshaft to Tranny-pan Clearance

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Transmission / Engine interface

Most automatic transmissions use a bellhousing that is integral (non-removable) with the main case. Either way, the bellhousing bolt pattern must match the engine or be made to match the engine using an adapter. A torque converter that matches the transmission is always used. 

The torque converter is then bolted to a flex plate.

The Flex plate must have the correct holes to bolt the torque converter to, and must bolt properly to the engine crankshaft. The flex plate also carries the starter ring gear, and must have the correct diameter and number of teeth to properly interface to the starter

The torque converter must also index to the crankshaft to properly center the torque converter. Non stock applications, such as a GM trans being bolted to a Jeep or Ford motor,  will require a custom adapter to make this interface.

In many applications, modified or custom adapter kits are available from High Impact. These kits contain needed bolt pattern adapters, and flex plate kits to bolt up and center  the torque converter in non-stock adaptations. 

In others, it may be necessary to drill the flex plate for the torque converter bolt pattern and machine a crank adapter to center the torque converter to