Alright, that helps quite a bit actually. I misunderstood part of your original post, it kind of sounded like you had a disk brake rearend in it from a later vehicle. But now that we know you have a 3/4 or 1 ton rear (could it be that your truck is a C20 or C30, perhaps had the doghouse swapped by a po?), that will determine what options you have. from what you describe, you've got either a 4.56 or 5.15 rear gear, not very good for highway driving. The easiest fix would be to swap a 4.10 third member from a '68-'72 Chevy, usually found in automatic V8 equipped trucks. Those are in high demand, and may be hard to find. Another option, if you're up to swapping the entire rearend, would be to use the Spicer 60/70 that was found under GMCs from '60-'72. Those are for the most part leaf spring rear, thouhg I have seen a few coil spring versions, too. The Spicer rear still has plenty of support in the aftermarket, just about any gear you could want is available for those. You couls also consider the 14 bolt Corporate axle, which was used from '73 to current, though you would want to stay away from '99 and newer due to width issues.
You might also consider an overdrive trans, though there really aren't too many out there that are an easy swap with enough stength for a 3/4 ton truck. Others might chime in with options there.
