[Dwight]

Has anyone ever mounted a FFR roll bar in an Arntz?

Maybe not.

My approach would be to measure from the frame to where the roll bar will pass though the fiberglass.
Next measure up the roll bar the same distance. Make a mark. Do this on all three legs.

Now take a piece of heavy paper (poster board) and make a template of the three legs. You can cut it up and tape it back together.

Now lay the template onto the car where the roll bar will be.
Mark the holes and drill the center with a 1/4" bit. This will give you room for error.
Use something like a wire off of a politician sign for a straight edge. Check your layout. If ok, cut holes with a hole saw or Dremel tool. The third leg will be egg shaped. Start small with the holes and you can enlarge if necessary.

I'm not good with the English language (or any language) but I hope this will give you an idea how to tackle the project.