OK, Having a newer truck, the CTS Type monitor will not work on the IDI's, even with additional work.
On the PSD's the engine uses sensors similar to the ones on the IDI's (Haven't looked, but wouldn't surprise me if some are the same since IH/Navistar built all of them) but, all the sensor inputs (EOT, ECT, PYRO, ICP, IPR, Boost, RPM, Volts, etc (I am sure there are some that I am forgetting) are all fed into the PCM. The PCM then uses this info the control how the engine is running, it also tells the Instrument Cluster what to display. The Instrument Cluster in the PSD trucks is just a display, the "gauges" you see don't "measure" anything, unlike the IDI's dash. The CTS Type monitors only show you what is already being measured, just not displayed on the factory dash.....
So yes, unfortunately, the easiest is to buy gauges. Or, build a monitor a mentioned above.....
Also, the only info you will get from the OBD1 port is for the E4OD transmission. No engine data is fed into a computer other than the Tach for the transmission control.....