You sure you have 4.10 gears? I'd bet 3.73's (that's what 99.9% of these trucks had). My one ton with 32" tall tires has 4.10 gears, and @ 2,600RPM I'm going a touch less than 60MPH (@ 70MPH I'm turning almost 3,100RPM). A truck with 4.10's and those short tires would be petering along at about 50MPH @ 2,600.
Also check out your carb and ignition; probably quite a bit left on the table if you're only getting 8-10MPG! That's just awful.
You can get a few $$$ for the Muncie (as long as it's not the HD truck trans with the really low first gear) to fund your potential trans swap, so keep that in mind.
Another potentially cheap(er) option to keep a four speed and reduce highway RPM would be a set of 3.07 gears (and maybe some taller rear tires). Your 60' time will obviously suffer, but for a daily driver that might not be that big of a deal.
I wouldn't worry a bit about the 350 not being reliable. You can also get MPG into the upper teens with one that's running nice in a half ton. The late model LS stuff is really nice, but the old 350 still does a fine job.
Personally, I'd be really tempted to drop a TKO in there if I had the money (especially if I wanted to act kind of stupid)...