Other than M/P-2 is there any class for 5.8L and under (350-ish) engines?
I assume you are talking about HSR?
I have just initiated a new scheme of "sedan" classes, S1 thru S6 which mimics what they did at Waterford back in the day.
S1 which runs in HSR group 5 permits period correct sedans over 5.0L.
However, the catch is that they must have the correct engine for the car, as manufactured. That means that your Firebird 400 is fine.
But....you will also need correct brakes and wheel diameters.
If you go beyond "period correct", then you are thrown into MP1 or MP2 (MP = Modified Production).