Dumb question here... are you running the 228 or another gmc 6 like the 270 or 302? I don't think a 228 would pull the 3.54 gears well in a 1.5 ton truck.
Those 5 lug rims are about 20 inch diameter with about 35 inch diameter wheels.
Using an online calculator with 3.54 gear, a direct drive transmission, and 35 inch wheels... 65 mph looks like about 2200 rpms. A 228 would not pull that, but a 270 or 302 wouldn't have much trouble pulling those gears.
I have a 1940 3/4 ton which I have mocked up a NV4500 transmission swap. IMHO, A NV4500 would be a much better swap than an axle swap... unless you are swapping the axle because it is worn out. I chose 4.11 gears to go with my NV4500 and 302 gmc motor and 30 inch tall tires. I should be able to do 70 mph at 2350 rpm
