If one of the engines is a 750, that would be my first choise on getting together. Especially if you use forged pistons. I think Mike's sells replacements for his 750 kit, they might work with your cylinders.
I read somewhere that after going to a 750, people think that the 650 should have been a 750 from the start. After doing Mike's kit I have to agree. It runs much stronger, and much smoother. Even with a stock cam. I think my 82 may get a rephased 750. The rephase is smoother yet.