Would leave tank in stock location for your '51, making sure it's clean, sealed and good sending unit. If under bed then you have filler hole in cab to contend with. I did something similar on my '46, ultimately went back with stock tank. Best of luck


It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed