I ran into the same thing, had an old Sun tach and MSD, they don't work together. MSD makes a Tach Adapter, you wire this in and then your tach will work. I tried this but must have got the wrong one cause it didn't work. My tach worked fine with the MP ignition. Easy fix, I installed a vintage Auto Meter and it works fine with the MSD. I think the adapter was $40-50 too. Here is a link to what the part looks like, you will need the right one.
http://www.ebay.com/itm/MSD-8910-Ta...Parts_Accessories&vxp=mtr&hash=item588c31e982