In tearing down several 6.5s (discarded ones from dealer dumpsters) I found that some were equipped with timing teeth on the front of the crank (4 teeth). These used the short hub style harmonic balancer.
If you are inclined to make your own tach circuit, check out the ATS610LSA sensor (hall effect). I'd like to use this sensor, and appropriate circuitry to make the OEM gas tach work in my '87 with the 6.5
I think there were 2 different ESS methods used. One had a module mounted on the vacuum pump (California emissions) and another used a hall effect sensor on front of crank. Maybe others, not sure.