Going to tall 16" tires and rims should help mileage some. I don't think 16" was offered on 1/2 ton during that time period but you might check a wrecking yard and look for 16" 6 lug steelies off a late model GM utility truck. I think the 6 lug bolt patterns are compatible but not positive. If you could run 31" inch tires (215X85X16 for example) that would drop your 60mph RPM by about 300 RPM for not much $$. Short of that, I'd be checking the local vintage bone yards for that ring gear carrier you need to run 3.55 gears.
-Ed
