a body or b body or f body manual box will work but will need a steering collumn adapter or the correct column from a manual car or lenghten your steering column (welding)i would bet magnum manifolds fit becus the motors sit very far fwd from the firewall in these cars.

ive gotten rediculouse gas mileage from slant six f body wagons before,it cant be too hard w/a 318.

is yours a roller cam motor?
what rear?