I have a couple of stock caps. I would want it back but if you send me your address I would drop one in the mail for comparison.
Here is what I would do. I would drive out the pin and push the cap down until it latches and check hole alignment. I would do the same with another cap to check alignment. If both caps give the same view I would take a small round file and work on the hole in the tank until the pin will pass. If you could get a drop of JB Weld on the part of the hole left open after the cap is in place it would keep the pin from moving when you open the cap. If the two caps show that yours is the problem, I would source another cap or file the one you have with the knowledge that once the chrome is scored you may suffer corrosion in that spot.
Russ