We did connect it to another 510 for comparison purposes, and the tach still read about 2X what it should have compared to a tach/dwell meter. As a last resort, we did try the adjustment on the back. There was a lot of adjustment available with only a small change in the adjusting screw position. We were able to dial it in and it is pretty accurate over a 3,000 rpm range compared to the tach/dwell meter. A good used speedometer was installed at the same time, so everything works on the dash!
Thanks for the tips!