The 59 Rambler link is very interesting, but a totally different car. The link points to a 59 AMERICAN. That's the small body car that is basically a slightly modified 1950 Nash Rambler. Nash made the 100" wheelbase Rambler through 1955 then went to a bigger 108" wheelbase body. AMC reintroduced the 1955 model with some slight revisions in 1958 as the Rambler American, which ran through 1963 with the same basic unit body. In 61 the car was restyled by changing all the external sheetmetal including the roof panel, but keeping the floor pan and all glass area the same -- the main unit body "frame" was unchanged except for the firewall. The 58-60 American look almost the same, and the 61-63 look almost the same, but unless you pay close attention to the glass areas you'd never think a 60 and 61 were virtually the same car!
While KozAZ didn't mention if his 58 was an American or the bigger "Rambler Six" (the bigger car was finally named "Classic" in 61, and there was also the extended wheelbase "Ambassador from 58 on too -- three models from 58 -- Rambler American, Rambler Six and Rebel [same as six but with a 250 cid AMC V-8], and the Ambassador [extended wheelbase Rambler six with 327 AMC V-8]). But he mentioned an enclosed drive. The big Rambler models, including the 1956, used torque tube drive (enclosed driveshaft, with the enclosing tube acting as the rear axle locator arm and transmitting all torque from the engine to the wheels). The American (and 50-55 Nash Rambler) always used an open drive shaft. The torque tube was used through 1966 -- 1967 and later big models all used an open drive.
The big cars have plenty room for a V-8, the American was literally built around that compact L-head six! The only OHV six that will fit the engine bay is the old 195.6 cid AMC engine (1956-65 -- not to be confused with the later model 199, which is a totally different and longer design). Even late model four cylinder engines are a tight fit in the 50-63 small body because the intakes stick so far out to the side! The best fit is a 60 degree V-6 like the Chevy 2.8-3.4, or the Ford 2.9 (I'm sure it was made in other sizes, but I don't know what they are). The only problem with those is the accessory mountings are usually to wide. The engine can be mounted a little forward to keep the accessories away from the "humps" in the inner fender wells of the small body, but even then some would have to be moved in a good bit.
Even the small 50-63 body is exceptionally strong! I have some photos of one with a built SBC that runs 9 second quarter miles using the stock front suspension and chassis. It has been back halfed to mount wide tires, but that is just connected to the original unit body rails. All pre 63 Ramblers (pre 64 in the American) have a box "frame" rail running from bumper to bumper welded into the floor pan, making them extremely strong -- just as strong (if not stronger) as a separate frame. The rails are rectangular and made of 18 gauge sheet metal (about 3/64" thick -- just under 1/16"[which is 4/64"]). They are then welded to the floor pan, rockers, and inner fender panels, making a very rigid and durable structure. The old L-head six weighs only 50-60 pounds less than a fully equipped SBC!
These old unitized bodies are built much stronger than later "three box" designs that don't have a common rail running the length of the body. All unit bodies made after 1965 or so use the "three box" design (engine box, passenger box, trunk box). Three box use the rocker panels for longitudinal support of the passenger compartment then have rails for the trunk/rear suspension and engine compartment/front suspension that overlap the middle box under the floors. The main tie-in with the center box is actually the side to side reinforcements built into the middle box floor and the inner fender panels that extend from the firewall and rear body out.