I have seen pics on this site of the hole cut to access the fuel pump. It really doesn't look that bad and seems pretty reasonable to do. With all yoru additional gusseting, that might be the best approach, once you confirm you need to do the pump.
First though, locate those pump wires in the frame rail and try to run the pump directly with 12V. If it runs, then you can rule it out and not cut a hole.
You said it cranks over fine but consider swapping the battery with a known good one just in case, maybe from your F450. Also, check the connections on the battery and make sure they are really clean.
You've got a bit of a stumper here....