Small blocks:
Since the small block was an option in the various M bodies swapping in a bigger, i.e. 360 for a 318, engine is a snap. Just be sure to use the correct motor mounts for the type engine your running. 273/318 engines use one style and 340/360's use a second style. The part numbers for the 340/360 mounts are 3642818/3817233 with wagons using 381722/3817235. The part numbers for the 273/318 mounts are 3642818/3817237 with wagons using 3817122/3817231.
If your running a slant six swapping out the kmember for a V8 kmember member will make this much easier. Since all smallblocks are externally the same you can run the later accessories with no problems, except for mounting the AC. 79 and later 4bbl intakes use a different AC mount than the earlier models. To tell the style you have look for a boss to the passenger's side of the thermostat. It's about 1 1/2 inches to the passenger's side. If it's there you have the later intake. If it's not there then you have the earlier intake. Another way to tell is by looking at the compressor off the donor car. If the compressor is the old RV-2 style then it is the older setup. The newer setup uses the radial type compressor that looks like a big cylinder lying on it's side. You can run the RV-2 with the newer intake as it doesn't care about the boss but the radial compressor uses this boss to mount the rear of the compressor. You will need to either run a torque converter with the correct balance weight for the engine you swapping in or use a B&M flexplate with the weight on it. MP sells higher stall, prebalanced torque converters for lockup Torqueflights so if you want to keep the later trans that's an option. If you want to run a 727 instead of a 904 Torqueflite either find a shorter driveshaft or have yours shortened. The stock axle in good shape ought to handle all but the most radical smallblocks. a 72 and earlier B body 8 3/4 rear axle will bolt onto the rear springs but the drive shaft length will be off by close to 3/4 off an inch too long. This could cause problems unless you shorten the driveshaft.
Big blocks:
Big blocks are a little easier in some cases and more difficult in others. First off there is no "bolt in" here. You will need to cut the engine mounting ears off your V8 kmember. Shorten them and bolt them to the engine. You will use the stock trans mount. slide the engine in with trans attached, bolt up the trans mount. This will locate the engine fore and aft. You will need to shift the engine about left to right to locate it. Make sure you have your exhaust system installed, use Hooker Competition or Super Competition headers for a B or E body if you want headers. Once you find the spot where everything clears tack the mounts in place. Unbolt the engine and trans and pull them out. Weld in the mounts and gusset them. The power brake booster won't clear an RB motor but will clear a B motor. If you want an RB motor either convert to manual brakes or use the booster out of an A body. The Copcar tandem booster might clear the RB motors so check first. Use the later waterpump housing that has the inlet on the same side as the smallblocks and you can run your stock radiator. Finding brackets to mount the newer (79 and up) radial compressor to the engine will be a trick as the B/RB's were phased out in 78. To the best of my knowledge there were never any big blocks equipped with the radial compressor and it appears the RV-2 will hit the hood if you try to use one of those. You'll have to fabricate the mount here if you want AC. Trans choices are limited to either the A833 or the A727. In either case you'll need to double check the driveshaft length. The 9 1/4 rear axle available in some M bodies will handle a big block, if in good shape. Conversely the swap to an 8 3/4 as outlined above is probably the best here.
Exhaust considerations:
Since the M body has never had a dual exhaust option you'll have a hard time running a full length true dual exhaust. My advice is to run a 3" single exhaust with a high flow catalytic converter and a performance muffler.
Transmissions:
Your M body will have one of three transmissions. The A833 4sp manual, the A904 3sp automatic or the A727 3sp automatic, both automatics are of the lock up variety. The A833 and A727 were not available in later versions (83 and up?) and the A833 was only available behind the slant six. If your swapping in an A727 or A833 in place of an A904 and your keeping the stock rear end then you can scrounge for a driveshaft out of a similar donor with an A727 in it. By similar I mean if you have a two door the donor has to be a two door and if you have a four door the donor has to be a four door. If your going to swap the rear axle for an 8 3/4 then just have your driveshaft shortened. If you have an 8 1/4 axle stock and wish to upgrade to the 9 1/4 axle then the driveshaft will not require any work just because of this swap.
Miscellaneous:
See the suspension section for chassis upgrades.
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