cheap/easy mpg boosters?

I've never had any luck with the slant, but, an engine is an engine and it should respond the same no matter, unless it is allready at it's max potential. Starting from top to bottom, front to rear, drivetrain to running gear;

Free flowing air cleaner with a K&N or equal filter. Open element or a MP air cleaner adapted on top, you get the idea...

Clean the carb up!

Being you want to change the slant out for a V8, I'll skip the intake and other parts for a "On the cheap" reply and suggestion(s).

The exhaust can just be enlarged. Going dual isn't going to help and I would be, it would actualy hurt. At 2-1/2 inch pipe size, it is plenty for the engine size and HP level of the engine.

An MSD or equal ignition with a careful look at the distributors timing is a good spot to find a few miles to the gallon.

IDK if a set of taller tires is up your alley, but it wold lower the Hwy. operating RPM of the engine. Inflate what ya got correctly, slightly more to oval the tires will also help.