Could be a bit trickier than that if the nut is curved at the bottom, to mirror the radius of the board (I can't remember if that is the case on early 70's P basses) but with any luck, it will be flat and a straight channel will be left for it.
Try using a piece of hard woood, 1" thick, wide enough to be broader than the nut and long enough to tap confidently with a small hammer, lay it flat on the fingerboard and give the end of it a gentle tap, making sure that the end that is pressed against the nut, is dead flush, (I hit one once that wasn't and I cracked the nut in the middle ).