copo-rat
Sorry to jump in here late.
The weight issue may not be so bad.
see:
http://www.carnut.com/specs/gen/chv64.html

The 250, warmed over, will probably work good especially with most any trans that is not a PG. Even a T350 is better due to the lower 1st gear.
You can swap pans and oil pump pickups on your engine but you still need a dipstick hole. The "spot" in the rear location is probably there but undrilled.

FWIW: My 63 Biscayne 4 dr (with a used 250 dropped in) would get 21mpg on the highway.

Pete

Pete
64 Chevelle
61 C30 Panel truck