In my -55 sec.ser GMC 1/2 ton truck I have swapped to a 12 bolt rear axle from a -75 4x4 Blazer. It has 3.08 gears and a locker. Truck has its original 248 and 4sp Hydramatic, and it has no problem with keeping up with traffic and cruises easy at 65-70 mph.
Only modification needed is to weld on new spring brackets. Be carefull and weld a little at atime, or the tubes will bend!! The orig brakes is a direct fit, so is the driveshaft, and it has the 6 wheel lug pattern. It is 1,5 inch wider than the stock axle, this is no problem when using stock size rims, but my 8,5 inch wide slot mags was too wide.
Look out for an axle from any year 4x4 Blazer or 4x4 K10 pickup. If you are running a trans with 1:1 high gear, 3.08 or 2.73 is fine. If your trans has overdrive high gear 3.42 is fine.
This is an easy swap that will improve yor truck for todays traffic.
