Overdrive or don't worry about it? !

Fuel injection preferably via a Holley system followed by overdrive with a .67 or similar overdrive ratio are the two best upgrades you can do for your car.

I drove a buddies car with a gear vendors on a 727, and while it's better than nothing, it's just not enough of an overdrive improvement to justify the cost in my book.

This assumes you've upgraded to at least front disc brakes, rebuilt/upgraded the suspension and bought some decent shocks. A US Cartool chassis stiffening kit is a big plus too.