Might look into the start signal. Going off memory here, but seems like some PCM’s need to control the starter relay and the start signal needs to go to the PCM rather than the relay. Not sure if there is a change to the tune to turn it off or not.
The other issue can be power to the PCM being interrupted when the key returns to the run position. PCM has to see power at all times. Might try jumping power to the PCM and see if it starts then.
Just some ideas.