[MrE]

Mike, there are numerous posts about the inaccuracy of analog gauges on this forum. For an inexpensive solution that will take you into the 21st century, buy J1939 sending units. Get a programmable BSM, a Bluetooth ODBII scanner, and an OBDII diagnostic port. Run a CANbus cable from the BSM to the OBDII diagnostic port, only CAN high and CAN low signals are necessary for this to work. Power and ground from a circuit controlled by the ignition to finish wiring the diagnostic port. Get 'Torque Pro', 5$ Android app. You can use your Android phone or tablet. Torque comes with a built-in GPS speedometer and compass. You can build your dash using your Isuzu, picking the gauges you need to monitor. If the J1939 sending units are sending a different identifier than the Isuzu, program the BSM to match.

I see that you can buy a Cat 3208 ECM, but have no idea what it would take to convert your current mechanical motor to electronic.