Fit won't be a problem -- the old 63 Rambler V-8 (327) was actually as big and bulky as the Caddy. It resembles a Ford 390 or Chevy 396 (The Rambler V-8) in size and weight. That should be comparable to the Caddy. The 195.6 OHV six cylinder in the 550 weighs about as much as a SBC -- it's no light weight either. For the front end all you need to do is swap in a set of Rambler V-8 springs. You can likely use the existing trans crossmember -- there are two sets of holes to locate it on the frame rails. You WILL want to discard the rubbe mounts on the ends of the crossmember though. They break down. Should be easy to fab a metal piece to replace it, or better yet fab a new crossmember and make some hangers that bolt to the original frame location. Do that and you can use the Caddy trans crossmember, just trim the ends to fit between your hangers.
The real problem is the rear axle. 56-66 big Ramblers (all except the American) use a torque tube. The tube encloses the axle and mounts to the trans, enclosing the single u-joint as well. The u-joint lasts forever enclosed like this! The tube also locates the rear axle. The axle is an AMC Model 15, which uses a 7-9/16" ring gear -- the equivalent of a Chevy 10 bolt or Ford 8". It's virtually the same thing as a Dana 35 used in a Cherokee. You need a 58-60" wide axle (hub to hub) to fit the existing wheel wells without tubbing, and then you can only run a 205-215 wide tire. The suspension is easy -- take the spring seats off the Rambler axle and mount them on the new one, then use the original springs. Ladder bars are the easy way to go, but use 36-42" ones -- the longer the better the ride. I made one with 26" bars and the ride was "choppy" on rough roads (like I-10 between Gulfport and New Orleans... glad I moved frm there last June!!). The best way to go is a "truck arm" suspension. A Chevelle kit can be altered to fit the Rambler easy enough, or make your own. A four link is doable, will just take more work. The shock crossmember won't support the weight of the car. If you go with coil overs instead of the original springs make an upper mount in the original spring seats.
One more thing -- don't "pry back" the axle to remove it. Disconnect all brake lines at the rear (park brake lines and the rubber line from body to axle), pull the bolt from the body end of the panhard rod, pull the four bolts on the torque tube to trans flange, jack axle up with a floor jack in the center pushed in from the back and remove tires, put jack stands under the body right in front of the wheel wells, lower jack, pull rear axle w/torque tube attached from the rear. You might have to get under the car and push the axle back with your legs to get the slip joint on the trans loose -- it's been on a while and doesn't move like an open driveshaft slip joint.
The only problem is you can't easily move the car without the axle in. What I do is take the tube off the axle next, then bolt a 4' 2x4 on each end where the strut rods and shock mounts were (take them off first, of course!), with a length of 2x4 across the ends (making a box). Then push this back under the car and lower it. If you're moving it on rough ground drill a hole on each side through the floor and run a 1/4" lag screw into the 2x4 to keep the axle from shifting when a tire hits a rock, etc. Can you tell I work on Ramblers??