The biggest problem with Rambler vehicles built up till the late sixties is they have a torque tube drive system. This requires a complete rear suspension swap if some serious hp. is going to be used. This is not simple as the cars are unit body and have no provisions for a defferent style rear.
I have built a 65 Rambler 770 coupe, with the "Twin Stick" overdrive trans and a Rambler 3:54 posi rear. I installed a small block Chevy, using a Lakewood Scattershield for a belhousing, this allows you to connect to the Rambler trans (Ford trans) pattern and then use a Ford clutch disc and a three finger pressure plate. adapted the clutch linkage to work. The motor mounts to the frame were simple "T" shaped stands welded to the inner unibody, there is plenty of room and a big block would probably fit, but the rest of the car would have to be updated and replaced to handle the power. As it was the stock posi rear is a tapered axle with a lock nut (similar to old Ford and Mopars) and tends to strip the splines. It made for a lot of looks and surprised a ton of people who thought it was a "Grannie" car. I actually bought the car from an elderly retired school teacher, who fit the part of a blue haired granny. Good luck, I love the odd rods.