I have been following this thread on and off since was first posted.... In years gone by, the rule about this stated that the rear seat(s) HAD TO be suitable for continued adult occupancy.... this wording NO LONGER EXISTS!.... It WAS simple then..... a 260Z 2+2 was the F/Production record setter in (IRCC)1976 - I think the "owner"/driver was Bob Stockman..... and the rear seat WAS suitable for an adult to sit in for an extended period of time....
In 1986, Gordon Hoyt showed up at Bonneville with a new car - a 300ZX 2+2 with a blown 259" Chevy..... His son-in-law set the C/FC record (that was in the era when blown engines advanced 2 classes) up to 241+mph....Our Vega with a naturally aspirated Chevy 358" motor was NO MATCH for that.... but we were running in Gas Altered though - setting the record in 1988 at 215.015mph....
Interestingly, in 1991, they separated ALL blown from unblown classes, the old HP Books Monza class record of 207+mph was reinstated and we ran the C/FC class that year.... I got into the 200mph club at 213.246mph that year and went home at the end of the meet with the record at 216.695mph...but with the Cohn's, the Pruitt's, the Batchelors and Doll/Fox/Christoffersons also running the same classes as us, the records did not stand for many years.....
The interesting thing is though, is that Gordon Hoyt's ZX 2+2 car WAS EXTREMELY AERODYNAMIC..... and unfortunately for all of us, it was VERY LEGAL to run as a coupe sedan by virtue of the wording of the rules regarding the back seat(s) at that time.... the 300ZX 2+2 body of that era IS, as far as I know, still a legal coupe/sedan body to this day.
.....but in a "run what you brung" environment like Bonneville, is it a fact that someone will come up with an advantage one way or another.... Gordon came up with his advantage by spending some serious coin on a near new car - the body of which fit the rules....
Incidentally, the Studebaker and Camaro guys ALL tried to cry "foul" when we showed up with our Vega for the first time.... In Classic Coupe and Sedan.... the smaller bodies STILL RULE!...OUR SPEEDS ARE still comparable to the modern coupe and sedan speeds....and in Classic category, with comparable drag coefficient numbers as an '81 Camaro, with 65% of the frontal area, the Studebaker and Camaro guys are still a way behind....
So, if we want to "level the playing field" (like NASCAR), we ARE going to have to implement "cookie cutter" type body rules..... Is this what we really want? .... I DON'T think so..... but, on this "grey area" stuff in regards to whether a car is a coupe or a GT car, it seems to me that a return to the "old" definition of the rear seat(s) being "SUITABLE FOR CONTINUED ADULT OCCUPANCY" WOULD CLARIFY this issue.....