If you have Excel you can use my calculator to tell you what classes you can run. Just enter the shipping weight, factored HP and trans type and it will tell you the legal classes and appropriate weights.
Remember that the rulebook says that for GT classes you have to use the shipping weight for the body style with the largest available gas engine.
www.employees.org/~bhracing/gt_class_calculator_1p1.xls
If you don't have Excel I can run the numbers for you.