From 1967 to 72,Chevy 3/4 tons (and I think 1-tons) with automatic, came with 4.10 ratios. The sticks came with 4.57. They're hard to find. Junkyards ask around $250-300 for the 4.10 pumpkin.
With 31" tires and 4.10's you would be running about 2670 RPM at 60 MPM. With 4.57's you would be running 2970 RPM at 60 which is on the high side if your 51 has the stock 216.
