msd, points, pertronix and tach
 

I removed the pertronix from my 75 and my sons 72 and installed points.
the tach bounced up and down in both cars.
I reinstalled the pertronix into the 72 and the tach works. The electronics must prevent any back feed.
I am installing a 123 distributor into the 75
Will the tach will work when using and MSD and the 123 distributor or do I kneed teh tach inteface? and if so which one would I use?