Small Block Distributor Recommendation

If you can't get close to 18 mpg on the highway with the set up you have, the tune is off. Vac adv only helps if you run the mixture lean on cruise.... that's the only place vac advance will help when hooked up to ported vac signal. I'd start by leaning down the cruise before wasting money on a new set up.

Don't recall rear gear in the car. Same basic engine with a 3.2 gear got 20 mpg with a 26" tire.

Use a junkyard dist/box with the timing locked out on the start up stand... that's a cheap way to do it!