rpm issue? electronic?

I would connect the + lead of the tach to the negative of the coil and the other to a solid ground. Don't know why the instructions would tell you otherwise since the electronic ignition is no different than points other than it uses an electronic switch to open and close the coil path to ground insteand of mechanical points. I have never seen any tach that hooked up any other way on an inductive igntion (points or electronic) + of tach on negative of coil and other lead on solid ground.

I would guess that you are right on the edge of getting a big enough pulse to tigger the tach with it attached across the coil and that would account for one car working OK and the other not.

Also, do you have the coil wired correctly? The wire from the ballast resistor goes to the + side of the coil.