The longer the thread gets, the more we go in circles !
Current ECM & tune - can only copy segment from a System folder from another tune of the same OS - not very probable as this is a proprietary GMPP OS (non-production). Means wiring a resistor / tach adapter.
Current ECM - reflashing with production OS - as you know, E38 & E67 don't actually get a full reflash - require an appropriate "base tune" to already be in the ECM - not sure how this would workout with GMPP tune. May have to have a "base tune" loaded in the ECM via a Tech 2.
The last option - different E67 with appropriate base already installed (from auto recycler) - then tune from there.
Those are all of the options I can think of - personally, I would just wire in the Tach - pick the "best" place (away from known sources of moisture) solder everything, and seal it in 2 overlapping layers of shrink tubing.
Regards,
Taz