Process for installing new closet flange on 1964 black tank

[PA BAMBI II]

I am getting ready to install a new closet flange on top of my above-floor 1964 fiberglass black tank. The old flange is long gone (thrown out during renovation), but I believe it was screwed to the top of the tank. The screws were mostly gone and were ground flat when the tank was re-gelled.

Can I drill and tap the inset cast metal fitting for new screws to attach the new flange, or is it better to glue the new flange on?

I am really trying to find out if there is enough (total diameter) cast metal inset in the fiberglass to drill and tap. I believe the process will be straightforward enough if I can ascertain if there is enough metal there to do it.

Thanks for any advice or insights.

Ben

[PA BAMBI II]

Cast fitting can be drilled and tapped. I drilled 5 holes and tapped 4. There was a spot where I only hit fiberglass when drilling...no cast iron.