Knowing that you did the 10K mod on an I-4 tach for a chebbie V6 and it worked... and that maybe it worked with another 3.4... emboldens me to try maybe a 30K or even a 50K trim pot preset at 10K, mounted where I can get at it, and give it a go. That should cover things. Thanks, guys!

I suppose it's working as a pull-up resistor maybe? I can see from the schematic that the 22RE ignitor is acting as a grounding input in the stock setup, with the other side of the tach on the +12 rail. Wish I had a schematic of the tach itself, it would answer so many questions.