Rear End Housing

Tom

To answer your question more directly, there isn't any "best" AMC 20 diff casting. All are pretty much the same. Do note that there is a difference in the torque tube (56-66 Rambler V-8/Classic/Ambassador/Marlin) and open driveshaft housings and pinion shafts. Other than the pinion gear and shaft parts interchange  AFAIK. The pinion shaft on the torque tube models isn't drilled and tapped for a yoke retaining screw and has a different spline and size on the end -- machined differently for the yoke of an open driveshaft.

I'm assuming you're going to use an open driveshaft model since you're building for SEMA and most likely using a modern trans as well. There have been a few engine/trans conversions that retained the torque tube, but it's a lot more effort than swapping to an open driveshaft rear axle and building a suspension. Not that difficult to weld in ladder bar mounts or use a universal four link kit with the stock coil springs and seats. To retain the torque tube you either have to make a crossmember with a mount to hold the front of the tube or do some cutting and welding on the transmission output shaft housing. Trans must be the correct length or the torque tube will need shortening too. There is also the possibility of the CV joint (only in 63-66 V-8 cars, 56-62 used a single u-joint) being bad. the centering spring and pin assembly wears out and ther is no replacement. Takes 40+ years to wear out inside the tube, but when it does the joint flops and causes vibrations. Not terribly hard to change to a single u-joint, but if I were converting I'd just use a modern CV joint. 

There is a photo of a crossmember mount here (scroll down):

This is the 50s Nash type mount that was used through 61 in the Ramblers with V-8. The 62-66 models use a simpler rubber cushion and four bolt flange (62 TSM shows a six bolt, but I've never seen one in the field, probably a prototype used for photos).

With welding the axle tubes, beefing spring purches and, installing one piece axles the AMC20 is much stronger than credited.

The narrowest which would allow for mini tubing is a AMC20 from a CJ Jeep. - not too hard to find. 

The narrowest bolt-in one would be a AMC20 from a V8 Rambler American which are expensive and not so easy to find.

I have a few AMC20's from early AMX and Javelin that could be used as is or narrowed and put in.  The spring purch centers are the same between the Rambler American and the early AMX and Javelins.