[Mike Carr]

Shipping weight divided by the horsepower (NHRA or IHRA) rating of the motor used = natural class the car can run. Car an also run up or down one class.

1984 Monte Carlo with the 305/180, shipping weight is 3,334 pounds.

3,334 / IHRA Pure Stock rating = natural class.

Assuming IHRA keeps the factory rating of 180:
3,334 / 180 = 18.5222. I/PS is the lowest class in P/Stk (Index is 14.55 seconds 1/4 mile and 9.45 1/8 mile), so you would have to run that. Minimum weight would be 16 (the weight break for I/PS) times the IHRA rating, plus 170, to determine the minimum weight, with driver, at the conclusion of a run. H/PS and I/PS would be the eligible classes.

I/PS 16 x 180 + 170 = 3,050 pounds.

Now, if IHRA rated the motor at, say, 205 hp for Pure Stock, same thing.

3,334 / 205 = 16.261. 16 x 205 + 170 = 3.450 pounds. Still runs H and I.

Once you know what IHRA rates the 1984 305/180 at, in Pure Stock trim, you will know your class(es) and minimum weight(s) for sure.