I find the Pertronix to be a pretty bad system(I've gone thru two)and know of three others that have had there Pertronix go south(pretty bad product)
I'm now useing a 75 240 distribitor,a GM ignitor(#d-1906)and a Accel
Electronic super coil.also have found that the AC plugs work great,there
cheap and they hold up for a long time.#R42FS gapped at .045.these
are one step down on the heat range(43 is normal)
hope this helps some of you.
price brakedown
Distribtor used $35.at pick n pull.
GM ignitor # D1906 New is $35./used $2.at pick n pull.
Accel Coil (#140008)$45.I chose this coil for reasons as follows:
Primary ohms+.5,secondary ohms 9k(just what the GM ignitor needs)
if your a cheapass you can use a mid to late 80's Toyota HEI coil round or square about $5. at pick n pull.
Wih his setup you can run AC spark plugs at a .045 gap #R43FS or
R42FS(colder)

Ben.