Plug the vacuum line off. Are you sure your Mech. advance springs are out all the way at 2500? Not sure why you want to do this that way, but it gets close if you have a good ear, steady hand and have the carb adjusted close. I would recomend maybe buying a cheap light in the future, or renting one from autozone or something to dbl check. You can check your initial timing afterwards to. Stock cams vs aftermarket timing changes initial settings and ends up recurving distributors.