1966 A100 with a Slant gas gussler. HELP!

I looked into this for my '66 A100 window..
Gm 200-4R is the length of a turbo350.
It's a 4speed auto ...was even used behind grand nationals. There is an adapter available to sb mopar...I'm running a 5.9 carburetor.
what kind of mpgs you getting?