The 50-55 Nash Rambler and 58-63 Rambler Americans are all virtually the same chassis. Nash dropped the 100" wheelbase Rambler after 1955. AMC was formed in 54 when Nash and Hudson merged. For the new AMC line-up in 1958 the company dusted off the 55 Nash Rambler dies, made a few minor exterior changes, and reintroduced it as the Rambler American -- the only time an old model has been successfully reintroduced in the US. In 61 the car was due for restyling. Only the exterior sheetmetal was changed with the exception of the firewall and dash. Glass area is the same except for the rear glass. Even though the 61-63 models look vastly different they are the same as the others under the skin.
There is only 24.5" between the upper suspension arm mounts - the narrowest point under the hood. A hump in the inner fender panel is right above the upper suspension arm mounts. This interferes most with installing a V-8 or V-6, and even most modern fours due to the intake (there are a couple exceptions -- Olds Quad 4 and Ford Ranger 2.3/2.5 with EFI -- those intakes wrap over the valve cover instead of sticking out the side). The only other I-6 that fits is the Ford 144-250, which has the intake made into the head (and therefore close in to the engine) like the old Rambler 196 OHV. All other more modern I-6s are just too long, including the 64+ AMC/Rambler 199/232. The old 196 OHV is reliable but parts are becoming expensive -- it costs as much or more to rebuild than any of the popular small block V-8s. The only other engine used by AMC in the little cars was a flat-head six.
The front suspension is strange to those used to ball joints, but strong and light. The old 196 six weighs just 40 pounds less than a typical small block V-8. It also has better geometry than a Mustang II type suspensions, though the Rambler could use a little more caster with modern radials (3 degrees is all you can get!). Order replacement springs from www.coilsprings.com
15% stronger than stock for the added weight and better handling (5% for weight, 10% for handling). Cars were sprung really soft back then due to rougher roads. The high mount spring makes the cars roll resistant, no sway bar needed as long as adequate spring rates are used.
Here is an article from a 1950s "Speed Mechanics" magazine and some photos of a 62 American drag car. Both retained stock front suspension. The 62 has been back halved and has a full roll cage.
The article mentions that removing the "humps" is controversial. This has been proven to be false over the years. The humps were there originally to support the upper shock mount, which was redesigned for 1953. The panel dies just weren't changed.
As you can see there is extra bracing under the hood. Power is a SBC with "over the rail" Chevy II headers through large cutouts between the firewall and spring towers, but only enough for the headers. I would like to see an AMC 360 in one with Jeep CJ "over the rail" headers...