Gas Tank in frame

[6863m]

I bought a special built tank from TEBOW in Florida to go in the 62 C-10 frame and it is 10" deep by 8" and I am a little confused by the different hook ups. I looked up sending units at Summit Racing and others and they seem to match the hole on the top of the tank. I also have a threaded hole in the bottom front for a fuel fitting. The sending units I have looked at seem to draw the gas from the top of the sending unit and a 3/8 tube. What is the hole in the bottom for if the fuel is drawn from the sending unit. Or is there a sending unit that is for the gage only and the engine fuel pump just draws it straight from the hole in the bottom front.

I hope I am being clear with my question. Also does anyone recommend a sending unit for my application.

Thanks.