I've used a Pertronix for years without a problem on my '70E. Black wire goes to the white wire of the tachometer, red wire from tach goes to neg. terminal of coil. Red wire from Pertronix, along with two brown wires, goes to positive terminal of coil. Tach works fine.

Some time back I did extensive experimenting with an MSD box, but could never get the tach to work with it, as it is current driven, an archaic system. In the process of experimenting, I accidentally, momentarily grounded the red wire from the Pertronix. I hope you didn't do that, because it fries the thing.

I kept the MSD Blaster coil, which requires interrupting the the red Pertronix wire/brown ignition wires with a ballast resistor. It also likes the plugs gapped a bit wider. I'm very happy with that arrangement.