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I have an authentic GT-C bumper on my FD- errr I guess I used to rock it. In short, yes you use the 99spec reinforcement bar. I'll follow you on IG and I can DM you pics from when I installed the bumper a couple of years ago.
These were referred to as the Mazdaspeed GTC or GT-C bumper over the years. So GT-concept is probably not going to get any hits. Rotary Extreme produced a copy of this bumper once upon a time. Should be a thread on the forum somewhere. Lots of track guys used it too so might be something in the Race sectioin
hth...