Installing a pos. grnd. Pertronix ('P') is simpler than neg. grnd.:
https://www.pertronix.com/support/manuals/pdf/ignitor12vpos.pdf
The black lead on the P needs to be spliced to your ignition switch wire. I don't remember what color the ignition switch wire is--IIRC, it's white, but not sure--but with points it should have gone to the '-' or 'SW' terminal on the coil. The black/white lead from the P now goes to the '-' or 'SW' terminal on the coil. The '+'--this may be labelled 'CB'--terminal on the coil now needs a wire to chassis ground; I used the white/black wire that goes to the battery cutoff switch. This will retain the 'anti-theft' capability of the cutoff switch, but the terminal from the b/w wire will need to be moved to the large terminal on the cutoff switch that (also) switches the battery to ground. Otherwise, you can just run a wire from the '+' terminal on the coil to somewhere on the engine or chassis that is well-grounded.
Timing will change significantly, but you should be able to start the engine after the P install, then re-time it. If you want to carry points as backup, use M/F spade lug connections instead of butt splices. Don't leave the ignition switch on without the engine running any longer than necessary; this will fry the Pertronix.