I moved the tank on my 46 to under the bed, just moved back to original spot. I had rigged the stock filler from side of cab, through floor into new tank inside frame. Had trouble filling the tank this way so be sure to have a filler door somewhere near the tank. Have seen little flip up lids in the wooden floor, etc.

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