My '85 was the same way with the MSD. I have an Autometer tach connected to the tach output on the MSD box, but wanted the stock one to work also. Put the tach adaptor on and they both work fine.
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1985 Mustang GT
'85 GT "Red302" 12.96@107.1 N/A ... Sprayin' for the 11's
2003 Torch Red Mach1 
'68 Fastback ...331 Stroker "Restomod"
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