If you have access to Microsoft Excel, download this spreadsheet:
http://www.employees.org/~bhracing/g...ulator_1p2.xls
You can enter the shipping weight of the car, which you can get from the NHRA classification guide on their website, and the factored HP of the engine you want to run, and the spreadsheet will tell you what classes you can run and what the minimum weights are for each class.