[Mike Carr]

Yes. Shipping weight divided by factory (or NHRA/IHRA HP rating equals the weight break for your natural class. Using my car as an example. 1992 Olds Toronado Trofeo. 3,444 pounds factory shipping weight. 170 HP rating from the factory, 170 rating by NHRA. 3444/170 = 20.258, rounded up to 20.26. That puts me in DF/S (D Front wheel drive/Stock). You use the 170, in my case, to determine the minimum weight in each class it's eligible for (which is C, D and E/FS). You can add/subtract weight to do this, either subtract weight to move to the top of it's natural class (DF/S), into CF/S, and add weight to drop to EF/S. Top of D is 19.00 pounds per factored HP. So 19 x 170 + 170 (driver weight) = 3,400. (170 is a driver weight, added to all cars, to determine minimum weight. If a driver weighs less than 170, they will have to add ballast to make the 'HP x weight break + 170 = Class Minimum Weight' formula, and can take out ballast if they weight more than 170) You will always use the NHRA/IHRA horsepower rating to determine Class and Weight, regardless of what HP the engine makes on an engine/chassis dyno.

If the factory shipping weight of your truck is 4,500 pounds, and you use the 320 HP crate motor, 4,500 divided by 320 = 14.06. M/CM is the lowest classed CM class, for vehicles with a weight/HP ratio of 12.00-12.49 pounds per HP. I believe, by rule, you can still run M/CM, but you would be at a pretty large disadvantage. You would be better off to use a higher-rated Crate Motor, to get your truck into the 12.00-12.49 range.