New Project: 73 Gran Torino

I have done basic to intermediate work on cars for years, but this is my first project car. I am looking for advice on performing a rebuild. Currently the vehicle does not run but I am not over worried about that, I know my way around troubleshooting a vehicle. What I am more concerned with is knowledge about if I can change the transmission from a 3 speed auto to a 4 speed manual, if I can find one for a reasonable price. Or how I should go about rebuilding the engine/ what I should change in it.

It has a 351 Cleveland, 2v. 3 speed auto transmission and a moderate amount of rust on the under body that I need to remove and prevent from returning. I will eventually need to give it a new paint job, front windshield and interior upholstery. Just looking for tips and advice from people that know more about this kind of stuff then me.