thickness of the body, live axle vs. IRS, insulation under carpet, heater vs. no heater, battery size, aluminum block or heads vs. iron, steel bellhousing vs. aluminum, drum rear brakes vs. discs, 2 rollbars vs. 1, T5 transmission vs. TKO, aluminum flywheel vs. iron, bumpers vs. none, variance in weights of the variety of wheels that are chosen, different frame configurations-some that are longer than the normal 90" wheelbase,
oil cooler vs.none
Those are just my initial guesses...