Sorry, but you're gonna have some welding to do. The "big" Ramblers (anything but the Rambler American) used a torque-tube drive setup 1956-66. Basically that means there is a pipe bolted rigidly to the rear axle (no rear universal joint) with a cushioned joint at the end of the transmission. The driveshaft is INSIDE the pipe with one u-joint at the front joint. The pipe (torque tube) locates the rear axle.
When you put a modern engine and transmission in, you have to lose the torque tube. That means nothing to locate the rear axle, so you have to weld something in AND replace the rear axle. 93+ Ford Ranger axles fit well (7.5" is good for a mild V-8 cruiser, 8.8" if you want to run it hard). Ladder bars are probably the easiest to use, just have to weld a front mount in. Universal four link kits are pretty easy to weld in also. Take a look at
rear end swap 56-66 Classic - The AMC Forum - Page 1. There's some junk in the thread, but also all the info you need. I'm available here or there for questions.
You don't actually HAVE to change the rear axle and torque tube. I have seen people make a crossmember that bolts to the car at the rear of the trans that the TT bolts to instead of the transmission. It works, but it takes some welding to make the crossmember, and a lot of measuring to get it in just the right position. The length of the transmission will mean a lot, and the front of the driveshaft may need modifying. On V-8 cars that's not problem -- they have a tubular shaft inside (front half tubular, rear half solid). The front of the shaft can be cut off and an appropriate end welded on that will mate to whatever transmission yoke. On six cylinder cars only some used a tubular shaft. 56-61 automatic trans sixes used a tubular shaft. In 62 automatic and standard trans sixes used the tubular shaft, OD manuals used a solid shaft (the exception being 4.10 geared standard -- they used the solid shaft). All 1963-66 sixes used the solid shaft. The tubular shaft cars can have the TT shortened easily if the transmission is longer than the original. The solid shafts would be difficult to shorten. They are made like axle shafts, hardened and splines on the ends.
1956-63 six cylinder cars all used the AMC 15, which is a 7-9/16" ring gear axle. It's fine for a mild V-8 in a cruiser with street tires that fit the stock wheel wells. If you're going to be running hard you need a heavier duty axle anyway, may as well go with the Ranger 8.8". 1964-66 sixes and all 56-66 v-8s used the AMC 20 8-7/8" ring gear axle. It's as strong as the 8.8" and even the 9" car type axle (28 spline small axle bearing).
The weak point of an AMC axle is the bolted on hubs. They aren't that weak, but loose tension over time. The axle nut should be loosened then torqued down to 250-300 ft/lbs (factory spec is 250, 300 won't hurt, but no more!). The pressure between the hub and tapered/splined axle is what holds the hub in place, the key is just to make sure the hub is put back on in the same place when removed to change seals or bearings. Don't mix the hubs -- once mated to an axle that hub MUST stay with that axle! Plenty guys race 10 second 1/4 mile cars with the stock axles. Drag racers generally retorque the axle nuts every season or every other season. If the car only sees an occasional trip down the strip and is generally a cruiser retorque the nuts every 5-10 years. It usually takes more than 10 years to have a hub spin problem, more like 20+ years (unless you have 500+ hp, with slicks and good traction control...).
