Centering the Gas Cap on a 68 Camaro

[Mrfixit]

I'm about to take on the project of centering the gas cap on my 68. I've seen several completed projects and think that the tail panel looks better when the lock is removed and the cap is centered.

By my quick measurement, the cap needs to moved upward approximately one inch. The increase in height shouldn't affect the filler neck but I'm concerned about the rubber sleeve. What did you do to compensate for the extra length so the sleeve would seal properly? Did you leave the sleeve out and make a rubber grommet to seal the filler/trunk? Make a spacer?

Thanks,