Custom fuel cell in stock gas location (01 Sierra)

OK, I need some help making my mind up where to place a custom fuel cell that I will eventually have built. Has anyone ever had a custom fuel cell built to replace the stock gas tank in the stock location? If so, what dimensions did you use. I know about raising the stock tank up but I don't want to do that.

Or, since I will be sheetmetaling my bed, what about just placing it inside the bed. I need some opinions please.