^^ There already is a Circle Track "NASCAR" class for ECTA.
To the OP, in ECTA, you can run 'up' in classes meaning that you need to find the lowest class that you will qualify, then you can run in a more 'difficult' class. e.g. if your car is D/FALT, you can run up in engine classes to C/ or B/FALT etc. or up in chassis classes such as D/FCC.
In reality, it doesn't matter what class you run in unless you expect to get a record. No official will tell you "you cannot run in that class" as you will not undergo record scrutiny until you surpass a record. You must, however, meet the safety rules for that class, including meeting all of the safety regs for the record speed in that class.
If in doubt and if you do not expect to set a record your first time out (very few do) then pick the class that, during speaking with Joe Timney or Keith Turk, they suggest to you, OR run Time Only. Once you run your car through tech inspection, have Joe or Keith walk you through everything that is right or wrong with your setup. It is a bit intimidating at first, but everything they say has a good reason and has your safety in mind.