Allan, I went through this process last month. YOU DO NOT NEED TO REPLACE YOUR TACH. OK, now that I've got your attention, use your working tach, just get the correct MSD tach adapter, 8920, and wire it up per instructions.
Here's a
link to my wiring diagram. And here's a
link to my website documenting the steps I took to get it to work correctly....
Good luck and let us know the results....
-- Rob