stock heads, stock turbo 400 converter then I would stay with a "near stock" cam.
Stock 6X-4 heads they don't flow air nowhere near the stock # 16 heads.
I have a 1968 Firebird coupe with a 428 +30 over - wide ratio Muncie and a 12 bolt 3.5 posi rear.
The heads I use are 1971 # 96 milled for a compression ratio of 9:1 with Magnum 1.52 roller tip rockers.
Stock heads only flow to about 4000 rpm. Check Out the Wallace Racing Calculators and shift points - very informative.
I have too much rear gearing in my 1968 Firebird. My latest Firebird project is a 1967 with a Mutt 455 again # 96 heads un-ported.
The 67 has a 3.07 rear and Big Injuns seem to love either a 3.07 or 2.73 rear gearing for a street car.
Good luck - Mutt 455