southerncharm,
I'm certainly not an expert on a '51 GMC but I have installed many conversion kits. The shift mechanism would be the same regardless of year, as long as it's a 3 speed. If the shift arms on the sidecover are the same then the only difference would be how the shifter attaches to the transmission.
While it 'might' not be a bolt on deal, it certainly could nearly be. If you are handy in the shop, you could make it work without much effort. Again, the shift mechanism is standard, the shift rods are adjustable and the shift arms probably fit, SO, the only 'maybe' would be how it attaches to the transmission.
If this is your only option, it might be worth a try. If you can find one listed for your exact rig (good luck) that would be the best try! These conversion kits are very very simple.
Stuart
