First skirts on the piston being smaller don't matter that much. If the motor is build right with the correct ring tension then the skirt will not hit the cylinder walls. Next you need to check the piston to wall clearence but DO NOT GO BY THE HELMS MANUAL. If you are running forged pistons the ptw clearence will be different then when running stock cast pistons. Also, is this motor going to be turbo, nitrous, or na. This will change the ptw clearence some also.