Originally Posted by oldbogie
331 Fire Power hemi with a Fluid torque 4 speed semi-auto would return this to original configuration. These are monsterously heavy cars 4000 plus pounds stark naked empty, a 251 Spitfire inline 6 will be aufly weak and since isn't anywhere close to original will seriously degrade the value of the car and the value of your work on it.
There's really two ways to go; one as I outlined above, will return it to collector car status; the other is a modern power cruiser in a classic body with a suitably sized V8 for it's mass, a modern transmission, disk brakes, modern ancillaries like alternator with 12-18 volt electrical, air conditioning, etc.
Op, i think this is one of those cars that you just have to restore to original condition. It's such an awesome car it would be a shame to alter it. Only thing is you will have a hell of a time finding a 331 hemi, so you may have to use a different powertrain
Do you have any pictures of the car as it is? Early 50's cars just have such an awesome classy look, i would love to have one if i could. You really have a cool project.