Help locating Blazer bed hole(s) for Fuel tank.

[Accelo]

I am getting ready to punch the holes for a Blazer fill in the bed. Can anybody help out with the hole locations and diameters for the holes in the bottom of the bed? I can best guess it but don't want to do that unless I have to.
The assembly manual gives only the fill assembly information. I am including a picture from a Blazer fill taken from the inside of the bed. It's a California 1972 truck.
I unsure what the exact reason for the round tube in back of the fill. I will not be installing that as it is likely a CA only emissions addition. I am only interested in what is under the rectangular cover piece.
Thanks