I think I would start with checking the PCM Power Relay and the Run Start Relay - for sticking, define cab replacement (meaning) whole cab was removed and all associated wiring was disturbed in the IPC and dash?
Do you have a - High End Scan Tool? that can read the relay circuit and the Ignition Switch Signals? Looking for ISP-R - Ignition Switch Position - Run or ignition Switch Status.
It may also have a code locked in.
I think you either have a short in the ISP-R circuit or a sticking relay which is keeping the relay energized and its not getting the controlled shutdown sequence or request, there has been one noted (same issue) where the BJB was internally shorted and the ISP-R circuit was constantly powered any other noted dash messages or faults while driving?
If what I am picturing is correct - all the cab wiring was removed and reinstalled , may be pinched somewhere