What ever you do, you will probably have to add brackets on the frame to attach to the mounts on the motor.

By the 55/56/57, you could buy a Dodge or a Plymouth with a V8. Those original parts may still be available, but how well they will attach to your mid 80s motor might be a good question.

I've not dealt with a 55 dodge car, 50, 54, and 55 Dodge trucks, but they have a different frame then the cars do. Does you car have a transmission mount at the rear of the transmission? Trucks do not.

Also, what are your plans for a transmission for the 318? The original trans would have had an emergency brake drum on the end of the trans. You will loose that, and you will probably want/need to update the rear axle as well.
There are web sites that deal with this specific era of cars, I'd Google search. Gene