If you only want to improve mileage, change out the final drive ratio to something more favorable. Say 3.50 to 3.70's...

Helping the engine breathe can be a dual edge sword in a lot of cases, helping the flow through the heads also can increase consumption and FE's have always been a hungry motor.

Using the "Other" motor I find replusive.. If you were going to build/rebuild the FE, find a 428 crank and add the edelbrock top end package, heads / cam / intake / carb... but you're gas consumption won't be going down as you'll need new tires more quickly also!!

And welcome to the club, nice to see you here!