To Connect your stock OEM Mopar tach to your MSD ignition box, MSD has a tach adapter that plugs into the Ignition box. (Part no 8910 or 8920) It converts the MSD signal for your stock OEM mopar tach.

A second option, there was a posting from a Moparts member, Mr Heaterbox, who was making new replacement circuit boards for OEM mopar tachs to work with MSD style ignitions. Search on this posting number: 5790084. Seems interesting but not sure how well it works.