Hi Twinvipers,
Been there done that, just last week. We're doing a custom project GTS and the owner purchased a BTR bar.
The mounting hole angle of where the bar comes down to the upper seatbelt area was straight, where it needed to be angled to match up with the factory threaded holes. Once we moved the holes in the bar mounting plates 1/2", it fit ok. Yes, we had to cut the fiberglass lip also. The lower plates to the floor needed to have the inside corners cut off so the seat bracket could be bolted back in and I think we also did something to either the holes or the plate to floor angle. That got the main hoop in.
To get the side bar to go in simply required cutting off the original end with the quick pin and changing the angle so the bar would come down on the forward end the 6" it needed to get to the floor. Then we cut the mounting tab off the plate on the forward end and changed the angle so the plate would sit flat on the floor. Then put everything in place, tack welded it, took it out and welded it on the bench. Then we finished up by having the side bar and mounting plate re-powder-coated red.
Oh, we also had to drill out some of the original holes to get the bolts to go through and had to grind some to get the side bar to slip onto its tabs on each end. Took about 9 hours.
Hope that helps,
Regards,
Sean