Tho I love the proposal, my nit....
(edit to eliminate brainfart, add 'displacement' in key locations)
...is the incorporation of 4WD and supercharging. I don't want to exclude 4WD &/or supercharging, I'd want to separate them.
Unless you intent to make this a demonstration class (no points, no trophies)(?).
Why not separate into H1 and H2 classes? The difference would be H1 is 2WD N/A, H2 is 4WD, G5-like displacement, and/or supercharged.
If the entered point/trophy Historic class is too small, you get dumped/bumped into the appropriate non-historic class (Production or G2 from H1, PGT, G5, or Open from H2).
This all assumes that more than two historic cars show up for a rally. I wouldn't mind coming in second to a Stratos.
Don't forget that the Mazda GTX will be 25 years old soon....