If you could find a couple 1969 fenders, usually even some totally rotted out ones will have the "ashtrays" still intact.

There are a few spot welds holding them on to both styles, cut (tiny belt sander) or drill the spot welds and they interchange.

I converted one Plymouth fender from a 68 to a 69 using a rotted out 69 "parts fender". Besides the side marker hole and the ashtray needing to be
changed, the '69 also has a small welded grille mounting bracket that needs to be swapped over.

It would probably save time to just fabricate something, and my hat's off to you for trying to install the pins in the right place.
(Can't tell you how many cars I've seen with the pins installed closer to the middle of the core support, nowhere near the factory location).

Good luck