OK, here's what I found. The king pin in my axle was pulled and measured at 5-3/8" long by 0.921 in diameter, shorter than the 5-5/8" length listed in the factory GMC Maintenance Manual but longer than the one used by the Chevy at 5-1/8".
Looking through the Chevy Parts Manual I happened to see that the '53-'54 1 Tons used a different pin than the earlier models, listed with a length of 5-5/8".
I spoke with John at Jim Carter Truck Parts and he was kind enough to pull a '53-'54 king pin set, part# ME1742, and measure. 0.921 by 5-9/16". He measured the length with a tape so it might actually be the 5-5/8" length that was the original specification for the GMC.
Either way it was close enough to order. I'll update after the king pins are installed. Jim