Sorry to disappoint you.... But I'm building this truck to cruise not to keep changing gears. I'll leave the changing gears to my Tow Pig (06 Dodge 3500 Mega Cab Dually 6 speed).
How were you going to control the upshift with the GM trans? On the Mercedes setup it used a modulator (same) and the bowen cable. Does this get incorporated with the GM trans somehow?
Shifts in the 700r4 are determined by fluid pressure and governor operation(output shaft RPM).
It also has a cable that controls line pressure, which is meant to be connected to the throttle, and should be pulled tight at WOT.