John, you DO NOT need a tach adapter. I have never even seen one, and I've built quit a few Shelby's with MSD boxes. I always make up a brand new wire and run it from the male spade TACH OUTPUT on the MSD box (red arrow below) directly to the tachometer input. You will have the green wire unplugged from the tach and in it's place will be the new wire coming from the MSD box TACH OUTPUT. Do not hook the tach up to the (-) of the coil.
