Yeh, pretty sure ANYTHING past "5" is a completely different deal. This 5 /5C seems to be a "switched" / inductive/ Kettering system just like the old Mopar box, while anything 6 or higher seems to be MSD meaning, "multi spark CD."
If you are unaware, CD discharges a large charged capacitor across the coil as a pulse, much like a camera photoflash, and there is NO 12V power on the coil. Coil NEG is grounded. You cannot hook a tach to a CD without special adapters or with some provision "in the box" for a tach output. If you move to CD (like an MSD 6 series) there must NOT by ANYthink connected to the coil except the two wires from the MSD box
