I was thinking about that. The problem is that the nut would need to be very thin as it acts as a spacer. What I'm currently thinking is that I'll cut the stud off and weld a bolt onto it.

If you weld to the piece you will need to be able to dress off the weld so it will sit right on the bracket. It may be easier to run a bolt thru the bracket, a small spacer and into the housing.

The problem is that the hex portion holds the tank onto the pump. If I don't have a good seal, I made have a leak. You make a good point about the bolt locating the pump. I'll make a point of it to my fabricator.