Hey Ramblin man, you won't have to big a problem, the 58-60 American has a tough chassis and front end, and no torque tube. Only the big 56-66 Ramblers had torque tubes, the 50-55 Nash Rambler and the 58-69 Americans used an open driveshaft.
You can't put a newer AMC inline six in as suggested. That car was literally built around the short 195.6 engine. You'd need another 4-5 inches to put the longer 199/232/258/4.0L in, and it just isn't there unless you want to take the stock heater out and cut the firewall up. A narrow 60* V-6 would be ideal. The only problem is the accessory mounts. You'd need narrow mounts such as used for hot rods to get everything in there. The humps inside the spring towers can be cut out for some extra clearance-- they are remnants of the original 1950-53 design. Back then the shock mount had a rod that ran from the mount to the "peak" of that hump. The shock mount was redesigned in 54 or 55, but the humps stayed -- why spend the money to make a new inner fender panel when AMC didn't need the room? So carefully cut the INNER PANEL ONLY and weld a piece of flat metal in. It will gain you about 3" clearance on each side. Then you can put in a 90* V-6 or even a small block V-8 -- but changing sparkers will be a b----h!
Look closely under the car and you'll see that though it is a unitbody there is a rail running the entire length of the car on each side, more or less an integrated frame. The easiest way to mount the engine is with one of the universal tubular crossmembers. The rails are 27" apart in the engine compartment in front of and just behind the suspension mounts, 20.5" between the suspension mounts. I wouldn't advise welding the crossmember in as that could weaken the metal. Instead, weld plates to the end of the crossmember and bolt it to the rails. There should be enough shims in the lower control arms to use a piece of 1/8" thick plate on the ends of the tubular crossmember going down to the lower suspension cross rod bolts. Then drill near the upper portion of the plates for a couple 3/8" bolts going through the rails and/or suspension mount brackets. You could fabricate side mounts over to the rails without a crossmember, but a crossmember is advised to keep the frame from spreading, something that has been known to occur over the years.
A new rear crossmember will need to be made, but that's relatively easy. Run angle iron across the car so that it reasts on the frame rails. Drill up through the rails and floor, then drop some 3/8" bolts down from the top. Once covered with carpet you'll never know. I would tack weld a fender washer up there, and the bolt to the washer to keep it in and from turning, that way you never have to lift the carpet.
You won't have to worry about the front suspension. The old 195.6 weighs maybe 100 pounds less than a fully equipped small block V-8 -- not much lighter at all. The Rambler suspension will hold the V-8 well. It doesn't have any anti-dive built in, and was sprung rather softly. I built one and ordered custom wound front springs 10% stiffer than stock. It handled like a roller skate after that!
Any more questions on the little Rambler, just drop me a line! I only check here occasionally... otherwise I'd have seen this earlier!