Although these engines are from the same family, they share external dimensions only. Left-to-right are the 390ci, 343ci, and 290ci blocks. A few '69 blocks and all '70-'72 blocks have the cubic inches cast on either side near the motor mount.
Engines
All 290-401 AMC V-8s after 1966 are in the same family. But just because the 390-401s have the same dimensions as the smaller 290-360 engines doesn't mean you can bore and stroke your way to any cubic-inch displacement with any block. There are differences internally. The most notable is the switch from 7/16-inch head bolts to 1/2-inch head bolts and the deck height increase from 9.175 to 9.208 inches in 1970. This is actually OK because most of the aftermarket parts we've found are designed for post-'70 401s found in fullsize AMC passenger cars until 1974 and in fullsize Jeeps through 1978, making them more plentiful than the early engines.
The 401 cranks and rods are forged and tough enough for a performance rebuild. Look for the 5.858-inch rods and the forging mark (left) on the crank. Builders have also been turning down the rod journals and using new 2.100 Chevy rods for performance 401s. | 
This is a stone-stock 401 from a '78 Cherokee. If you know what you're looking for, it isn't too difficult to score in the boneyard. The engine builders we spoke to told us building 450 hp at 6,500 rpm is no problem if you modify the oiling system to supply the No. 4 and 5 main bearings. | 
After 1970, the 390/401 engines had dog-leg exhaust ports. Early text suggests they flowed 50 percent better than the rectangular design. This is more for identification since we would probably use modern aluminum heads. |
This is the Model 20 that you get with a V-8 AMC. It's got a bad reputation, but with the right parts it will work in a healthy street machine.
Rearends
Here is the problem with the Model 20. The axle passes through the hub and is attached with a bolt. Fixes include replacing the hub every time the two are separated, following the complicated factory setup procedure, adding extra Woodruff keys, or replacing the axles with a new one-piece set. We fear all but the last fix.
Most of the AMC V-8 cars were saddled with the ill-reputed 87/8 Model 20 rearend. The problem has always been that the axle is a separate component from the hub. To make matters worse, the axle uses a Woodruff key instead of splines, creating a potential to spin the axles in the hub and snap the key. Short of changing rearends, the fix we've seen used universally is a Detroit Soft Locker, Auburn differential, or similar design with a Moser one-piece 29-spline axle to put up with the abuse. If you're going racing with 500 hp or more and launching hard on slicks, there are Dana 60 and Ford 9-inch bolt-in housings available. Otherwise, upgrade the 20 and leave it alone.