Cheapest way to change ratios on an 8-3/4, and what ratio to choose?

Here's a dumb question fer ya. Please indulge me.

When I built my Valiant, I opted for a 3.73 Dana Power-Lok posi diff. But now I'm getting a little tired of my engine having to rev so high on the freeway. So I'd like to replace the ring and pinion with a higher overall (numerically lower) ratio, but after spending so damned much money on the Power-Lok, I want to go cheap!

The car is a '62 Valiant with a 408 having 10.5:1 compression and what we used to call a "292 degree" cam (before the days of measuring duration at .050"). ThermoQuad carb.

How can I get into some new ratio cheaply, and what ratio would you recommend for lower RPMs and better mileage? It can be an open diff. It doesn't have to be a posi.