You've got the bigger 58 Rambler Six (has just Rambler nameplates), not the small 58-60 American (has Rambler and American nameplates). You won't lose the heater. The American doesn't have a torque tube, only the 56-60 Rambler Six, 58-66 Rambler Ambassador, 58-60 Rambler Rebel (same as Rambler Six but with a V-8), and 61-66 Rambler Classic (finally gave the Six a real name!) have the torque tube. The bigger Rambler body came with a V-8 option from the factory and has plenty room for a V-8 with no cutting under the hood or losing the heater. The small American doesn't have to lose the heater unless you use a Chevy engine -- the rear distributor causes loss of heater. A 302 Ford or 318 Chrysler is a better fit, any small block with a front distributor (or newer model with no distributor).
As for the bigger car, all you need to do is fab engine mounts. The crossmember doesn't have "perches" for mounts as the old cars used four mounts -- two in front of the engine and two on the bell housing. The trans just hung off the bell, even the big heavy autos back then, which wasn't a problem. You will need to fab and weld or bolt perches to the crossmember or devise some other means to mount the engine, and make a rear crossmember for the trans. Trans-dapt and others make universal crossmembers for hot rods that will work for the engine and trans if you want. J.C. Whitney carries them, but their site search makes them hard to find (hell, anything is!!). Try different combos of phrases until you do. The distance between the front rails in the 58 is 31-1/2".
The torque tube is the main locating arm for the rear axle. You will need to replace the rear axle with one from a 83-92 Ford Ranger. Only the 90-92 4.0L models have the 8.8" rear axle, the others are 7.5". If you're building a cruiser with moderate power (400 hp or under) the 7.5" axle shouldn't be a problem. The Explorer and 93+ Ranger axles are 2" wider than the earlier Ranger. They will fit, but you will need deep set 4x4/front drive wheels to fit in the wheel wells. The stock wheel wells will take a 215 tire easily, a 225 will fit but might be snug.
Since the tube located the axle, you have to have a way to do that once the tube is gone. A four or three link kit will work, but ladder bars are easiest. Just make sure the bars a 30" long or more -- the longer the better. A "truck arm" kit for a GM G-body from "Hot Rod to Hell" (http://www.hotrodstohell.net/truckar...karm_gbody.htm
) is easy to mod to fit. All you need are the arms and crossmember they mount to -- the panhard rod from the Rambler can be used. Just grind/drill the rivets and bolt/weld it to the donor axle. The spring seats on the Rambler axle are easy to remove, they are hold on by one 3/8" fine thread bolt in the center. Just place them on the new axle and drill and tap it for the bolt.
Removing the Rambler axle is easy. Disconnect all brake lines and the shocks. Unbolt the tube from the transmission. Put a floor jack under the car from the rear, jack up the axle, and remove the tires. While the car is up put jack stands under the body in front of the axle. Lower the jack then reach under and pull out the springs. You might have to jack the body up just a bit more, but with the tires off the axle will usually drop enough to get the springs out. Pull the axle out the back of the car by rolling floor jack back. You might have to get in front of the axle and give it a good kick (good jack stands, chock the front wheels!) as the yoke doesn't slip on the trans and has been on for near 50 years... it tends to stick.
The only drawback is you can't move the car with no rear axle, and with the torque tube or transmission out there's nothing to hold the rear axle in place. I bolted a 4' length of 2x4 onto the axle (on what would have been spring pads on a leaf spring car) and set the body down on that -- with the gas tank out too. If you hit a hole or big rock the axle might shift, so be careful if doing something like this! A friend used a pair of furniture moving dollies on casters with jack stands on top to move his around the shop.