if yer talkin about a TF truck, what yer missing is a tank the same dimension as the one behind the seat would be dragging on the ground
in order to have the stock fuel pump be able to pull the fuel the bottom of the tank needs to be about level with the pump, or slightly higher
do some searches, folks have talked about tanks they've built or adapted, and also about
the lack of a compelling reason to go to the trouble and expense .... the stock tank works fine
Bill