Not this equally relates to your situation but I have older fiberglass front fenders on my '57 and stock trim rings come nowhere near fitting. Rather than working the fiberglass to fit, I used what I had available at the time: a couple of '50's Willys (Jeep) pickup headlight buckets and trim rings. The trim fits just right into the recess. It's different but that's ok with me. Hopefully you won't need to go to my extent and get yours resolved.
