Re: hooking up a Crane Hi-6 and LX92
The factory tacho gets its signal from coil "-". There should be a pink and light blue wire in the loom heading back to the firewall from the coil. Just cut that one and connect it to the tacho output from your HI-6 (green).
What else do you have coming off the coil +? There will still be 12V at those wires from the ignition so that shouldn't effect those other "accessories?"
Looking at Figure 9 (Mallory Unilite hookup) this looks very similar to the way you'll do it with the Ford electronic dissy. With only on small difference to the way an MSD wires up...........
Reading thru the installation instructions, you need 12V ignition to the small red that goes to the dissy. So you may need to splice in a 12V ignition wire from the old coil +. See p16 second last para (LED status light when ignition on etc). An MSD has a separate red wire going to the ignition and 2 separate wires going to the dissy only.
Hope this helps.
393 Tremec TKO 9"