Father/Son Project: Build Ford Big or Small Block?
Build something he can afford to run and insure when he turns 17 or whatever the age is in U.S.
Pick something which parts will be readily available and cheap for years to come.
Having changed a c4 for an AOD in my v8 Ford Cortina here in England I would recommend going with an overdrive transmission from the off. AOD or T5.
If I was in America , would go for early fastback mustang if cheap enough. Here in England they are twice the price of coupes.