The final thing I needed to get to the Racepak: RPM. The ECM has no tach signal output. Autometer tach signal adapter part number 9117 was the solution.
Pic here shows how it’s wired in, this was just temporary crimps to verify it worked before wiring it in properly…
The signal adapter gets wired in-line with the two main +12v supply wires that go to the coils, one wire to each bank. Follow the instructions included for an LS engine but your wire color is different. Pink/black is what you’re looking for in the LFX harness. Start at the coils and follow the light gauge pink/black wires up-stream until you find where they split out from a thicker gauge pink/black. That’s the wire to intercept. There are two of these wires, one for each bank. The autometer box outputs a lovely +12v square wave standard tach signal. The Racepak instantly reads it. Change the Racepak to V6 Even Fire (default seems to be V8) and voila.