Do I need to retain the factory engine computer to run the trucks gauges?

Yes. The PCM is retained so that it can communicate the tach signal and vehicle speed to the instrument cluster. HP Tuners or EFI Live can be used to reprogram the PCM, turning off all unused/unnecessary tables and the check engine light. The factory GM oil pressure and temperature sensors will be installed in the Cummins engine. Coolant level and oil level can be retained if desired.