Try this.
http://transmissioncenter.net/subject.htm
This one gives setup for Diesels.
http://www.tciauto.com/Products/Instructions/instructions/gm_tv_cable_adjust.htm
The one in the water crossover is the one PCM takes its readings from. If it goes bad , it will cause driveability problems. The one in the head is for the water temp gauge. It will cause erroneous temp readings if bad.
2850 is for the P-series. The only difference is the "mounting height" (the distance from blade mount to end of nut for the pump). It will set the blade back toward the engine about 7/16 of an inch.
2831 is for the 6 blade fan and 4 bolt water pump flange.
2839 is for the 9 blade steel fan and D-MAX fan with 4 bolt water pump flange.
2840 is for the 9 blade steel and D-MAX with screw on pump.
Hey TD. Did you go with the dual thermostat setup or the single. I thought I remembered you saying you were going to try the single, but am not sure which way you decided to go.
The Hayden severe Duty Clutches (what Heath sells) are designed to engage at a lower temp than the stock clutch. The Hayden book says they engage at 170* radiator air temp (about 200* water temp on efficient air flow).
Why not swap the plastic fan to old clutch to see if it works.