I hope your also changing out the puny 7-1/4 axle to something stronger or it'll go up in a cloud of smoke the first time you hammer it. That will also change how much you need to cut down the driveshaft. In the past I have always done the method like KrazyKuda described and it worked fine but some shops nowadays want it measured their way like Supershafts recommended so like he said check with them first.
