I would say try to find a motor as complete as you can and then try and run everything that came with it (i.e. brackets, intake manifold, exhaust manifolds, etc). I dont have any experience with the 50's trucks but I would try the stock parts first to try and save money. You might be able to trim your problem areas a little and then box it back in for security.
I understand the "wanting" the car manifold for looks but the truck manifold can be cleaned up. Picture from
this build thread. But the truck manifold definately will work better in a truck that sees some towing.
On you torque converter question, yes you can use the stock. In fact you would not want a higher stall for towing. It would slip more and generate more heat.