71 rear end questions - changing gears?

[Brently]

I have a 71 with a 3:42 posi rear end in it - the clutches seem to be stuck because the left rear wheel pops in a slow turn - it seems to be to low of a gear for the 350 / 350 TH in it - although this is the rear end and motor and transmission that came in the truck - it has great take off but winds on the highway - i am very rear end ignorant - what gear ratio do i need for highway traveling that will lower the rpms and save on gas and from what vehicle year and make do i need to get it from? - HELP!! Brent