rear end, engine, transmission combos? best engine rpm for mpg?

[SATX]

Hey everyone,

I'm in the process of building a '67 SWB with a 350 from a '79 ambulance and a 700R4 transmission. I have two rear ends sitting around. There is a 4.11 limited slip and a 3.73 open differential. The rear rims that came with the truck are 15x10 Convo Pros.

According to http://www.csgnetwork.com/multirpmcalc.html, a 295/50/15 with a 4.11 rear end and overdrive puts the engine rpms at 55 mph at 1991 rpm and 70 mph at 2534 rpm.

The 3.73 rear end puts 55 mph at 1807 rpm and 70 mph at 2300 rpm.

Should I buy 16" rims in order to have more options in tire sizes for the 4.11 rear end? Which rear end is the better option for this engine and transmission? What is the sweet spot for rpm's and engine efficiency? All advice is welcome. Thanks for your input.