Well, I still feel the axle is up to the job however fast you end up turning it, and think about it, it's only the pinion that turns faster, the carrier turns slow, in fact extra slow because of the large reduction.
Let me restate that, I think the rear axle will hold up fine, but you'll have a difficult time getting the speed you desire out of those slow gears with whatever transmission and engine you want up front. The gears are just to low to do what you're asking, and that's the problem, not durability.
Switching to a a 4BT does not help your current gearing plan, it would be wound pretty tight at 2500, but you could do it no problem.
More realistically 1800-2000 rpm is what you want for the 4BT, should see reasonable mileage there.
The smaller RPM range of the diesel would make the 4 speed less desirable, the jump from 3rd to 4th would seem even worse.
And to overcome the slower operating speed an OD transmission will be helpful.
If you want the 4BT think about buying a whole P30 with the 8 lug I-beam front axle (to narrow and swap) and a 14 bolt GM rear.
The rear probably comes with 4.10, and faster ratios are easily available.
You may not want a cab and chassis rear axle (probably comes in the P30), check that the dual wheels still fit and don't hit the springs. If that won't work a pickup truck dual wheel axle is a little wider and will fit.
First pick your engine, then plan the use for the truck, and calculate gear ratios and speeds from there.
Check out
www.4btswaps.com and you may find this one interesting
http://www.4btswaps.com/forum/showthread.php?t=3731 although I still think axle swaps are a better idea than a frame swap in most cases.
Grigg
