Moving Gas Tank?

[Ric2L]

I recently bought a bed that has a hole in the side of it 18" from the front of the bed on the left side for a gas tank filler neck. there are straps under bed also. does anyone know where i can get a gas tank to fit this bed or one that would fit, maybe 73 or newer truck? I would really like to eliminate the tank from inside the cab and use a tank under bed if possible, but i dont want to put the filler neck in the bed floor.

thanks Rick