Class 6 would be things like Renault Clio's and Ginetta G40's, so essentially things about 6-8 seconds slower than an S2000 on the average race track length.
Regarding Trans-AM cars I would be inclined to put them in Class 4 as you said. The NASCAR's and the Aussie V8's are for more difficult because there relative pace compared to say, an S2000-Spec Chevrolet Cruze or BMW 320i, is very different from track to track. For example at Le Mans they would be streaks ahead because of the long straights that track has, but I am also inclined to think that a V8 Supercar would definitley be quicker than an S2000 at every track on the calendar. NASCAR's aren't exactley my speciality but I would say that both maybe best matched on terms of pace in Class 5.
I left Grand-AM until last as they are easily in Class 7, infact the britcar Class 1 I was reffering to has GT2/GT3 cars running alongside an Aquila and a Rapier SR2. But if you look in the regulations for Britcar they are infact a sub class within Class 1 known as LMPX which could also cater for (I imagine De-Tuned) Pre-2005 LMP2/675 cars as well as Grand-AM machines.
So essentially Class 7, which is noted to possibly be in Open Class or Class 1, owes it's insparation to Britcar's LMPX sub-category.
Now the Main classes I was thinking of at first were 1,3 & 5 but I thought I would dish up the other ideas in my mind too.
Wow, wasn't that a mouthful!
Adam.