I have not done the conversion yet, but will probably just buy a 12-20 gallon fuel cell, mount it under the bed where the spare tire used to be, and have the filler come out of the bed floor in the back. Probably use a nice chrome filler door to hide it all, and weld the hole shut in the passenger side of the cab.

I am assuming you are talking about a 49-53 style truck.

1950 Chevy 3100 5 window "in progress"
1967 Chevelle 300