When I added a factory Rally Gauge cluster and in dash tach into my GT-37 from the other 1971 GT-37 I had, I just routed the tach wire through the same gromet the vacuun line for the A/C used. It came out in the center of the firewall very close to the distributor.
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1971 Pontiac GT-37
Car is a junk yard dog and maybe one day will be restored.
