I have what's left of a '49 or '50 Chevy 1-ton where the guy replaced the original rear with a 14-bolt Chevy with 3.70 (?) gears. He said that the stock 216 would no longer pull the truck.
I assume your GMC is a 228, but that's not a lot more motor then the 216, so with those gears you might have some trouble.
On the other hand, I have a GMC 270 and SM420 in my '55 1st Suburban with 3.08 gears, and while its not a race car, and struggles a little pulling a loaded trailer, just driving it around it does just fine.
