Does this ride have the original motor and power train?
This car will be worth a lot more money with the original engine and power train.
Flat heads are simple to rebuild.
Keep this in the back of your head, 30's plymouths are pretty rare.
If you go V8 you will need to build a complete chassis to handle the power, I would start with a modern truck frame.
And yes to the above posts 318 will make a smooth cruiser.
Look into dakota trucks, the frames fit older bodys decent if you are good at fabricating.