The mean green machine sounds great!
I grounded my tach center post because it was not mounted to the tins/dash which should supply your ground. It wouldn't hurt to run a temp wire to see if it helps. It appeared that it was working very little. Perhaps it is getting too much signal and may require a filter? Mine in my build thread is an original and is hooked to an HEI w/o a filter but it is slow to respond and I'm wondering if the performance could be better with less signal?
It is an 8cyl tach right?