Jim gave you the easy way if you saved and know which lead from the old unit ran the tach. Otherwise you will need to run your MSD lead through the firewall and attach it to the tach. I don't know how difficult it may be to get to the back of your tach while it is in the dash. Also if the tach should not read correctly you may need to get the small converter unit that some people have to use with a MSD unit. But not all have to so I am not really sure why some do and some don't . Possibly the brand of tach.
Ron
