307’s are ok. Guys used to chooch up 283’s so why not?
Going to upgrade trans, a 700r4 can be had just as cheap as a turbo 350. Then you may see mileage gains. And can play with gearing later if it suits him.
This summer we are pulling a 97 vortec to go in a 69 gmc. With the overdrive the lower gearing will be a bonus. I think it as 3.73 or 3.23.
I forgot exactly what.
Yours sounds like a funner project.