If your rear Burb fenders are otherwise in good shape
I would definitely repair the edges near the bolt holes.
If these areas are rusted away then patch them
with either metal or fiberglass as you've noted.

If they are just "thin" then reinforce those areas with Fiberglass; right over the holes and re-drill the holes. Treating thin or rusted areas with POR 15
is a good plan pre or post repairs as necessary.

Wrestling with aftermarket reproduction Panel/Burb fenders
is an exercise in frustration so keep your rears metal!
IMHO
Squire