So I have determined that I had the completely wrong application for my speed sensor off of the 727. I was using a part off of a Wagoneer which had the 7/8 input and the 5/8 output. The problem was that it didn’t output a true square wave and required a converter. But the converter couldn’t keep up!
Now on to plan b -
AutoMeter 5291 AutoMeter Programmable Electronic Speedometer Senders | Summit Racing
And a new speedometer cable to replace the incorrect one.
I could use the GPS module for the Holley Terminator, but I’d really prefer to have a speedo that won’t blink out in a tunnel!