A 350 will do fine. Now take a deep breath and start over. You need an overview of what YOU want this 53 to be. Ask yourself and us questions and give us more info.
1. What rear end ratio do you have now?
2. Are the motor mounts really for a 350?
3. Do you want a third pedal or auto?
4. Have you got access to an engine and tranny or are they already in the shop under a tarp?
5. And so on.
Good luck.
