[oldopelguy]

Tall, skinny, properly-inflated to slightly over-inflated tires are the first place to start. Second place is new top quality synthetic lube in the trans and rear axle. Third is top quality synthetic grease for the front wheels and u-joints.

It might be worth slipping a quart of type-F ATF in with the new tranny fluid to firm up the shifts some. That's an old trick for firming up GM transmission shifts, though without knowing the condition of your trans it's hard to know if it would work well, not work, or damage something.

I would suspect that for the best possible fuel economy you would want to use the EFI manifold and injectors and such but swap to an aftermarket computer like an SDS unit or a Megasquirt that runs off either a newer hot wire MAF sensor or a MAP sensor. A more sophisticated injection system should allow for better fuel economy if it's dialed-in correctly, or at least more tuning options to get towards more mpg.