The 1967 car they got down to 2550 lbs that they were told not to bring back, they kept running through 1968. They just swapped the nose so it looked like a 68 lol. They would swap number plates to run the cars through tech to avoid rolling thr lightweight car over the scales. Even after all the teams got caught acid dipping by 1968, they just slowly reintroduced lightweight parts over time to get the cars back where they were by 1969. Even in 1970 the Autodynamics Challenger driven by Sam Posey got nabbed dipping. That was part of the popularity in vinyl roof coverings on the cars, to hide the oil canning. Sam talks about it in
this interview - start around 2:20 and then Sam begins talking about his car at 3:00.
Attached are a couple of excerpts from
The Unfair Advantage detailing some of their efforts in getting the lightweight cars through. Apologies for the cell phone pics of the book lol
By 1969 the SCCA rulebook had the trans am cars at 2900lbs, though as you'll see in Sam's interview I linked, they were running much lighter cars in the races. Iirc the Penske's secondary car was around 2700lbs, not quite as light as the primary car. Nowadays SVRA rules put the Camaro at 3000lbs for historic trans am, but they're well over 500hp.
I think CM are closest to real life but CS are probably the sweet spot for the most drivability.