you may be able to pick up a half mpg or so, but not much more. We find this out with our Revcons. For the front wheel drive, Revcon used a transfercase with the driveshaft running forward. The transfercase has a 1:1.11 step up gearing. Since it only takes about 2 hours and some basic tools to swap the gears, this was a common practice amongst many Revcon owners. The report was that you only lost about a 1/2 mpg by doing the swap. That put the engine at around 3K RPM at 65, similar to what you are talking about. So while you could make the change, the savings will take a long time to pay for its self. On the other hand, what is more likely is that you will still turn the same RPM, but just drive faster. Ask me how I know.
'83 Revcon Prince 31' FWD FMCA F298817
502 w/Howell/GM ECM/Edelbrock MPFI,Thorley's & Magnaflows, 4L85E 4 spd, KoniFSD