Gas Tank question

[56SS]

I know some of you guys out there have made your own gas tanks to mount under the beds, so I figured I'd see if anyone was willing to help me out.

I am looking to make my own tank that I want to fit the 2001 5.3L fuel pump in as well. I want to mount my tank right behind the axle, I think. I have a friend who is going to help me with some of the fab work, but I'm not sure how to go about my design and what measurements would be right. I believe the fuel pump is ~9.5 inches long, but I've never made a tank before, so I'm not really sure where to begin. Thanks in advance!