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

I think you've answered your own question. If the purpose of wanting to regear is economy, pretty much, it ain't gonna happen. Now an OD transmission on the other hand would help, but IMO it's not gonna help enough to justify the cost.

As far as the higher compression comment, efficiency goes up with compression. Always has. Of course so does the cost of fuel.