I recently rebuilt my 1996 GMC K1500 5.0. Everything is new electrically except the PCM. Distributor, cap, rotor, camshaft sensor, ICM, spark plugs, wires, solenoid, spider injector assembly. Crank shaft sensor, knock sensor, ignition harness, all O2 sensors.
I replaced rod and main bearings all seals and gaskets. I did a compression test on each cylinder after reassembly and all checked out.
when I took it out for the first test drive I got the P0306. I suspected injector issues based on my scanner. I took it to a shop and they confirmed. That’* when I replaced spider injector assembly. Then I got a P0300, which was a vacuum leak at the injector plug and the upper intake housing. I reseated upper intake and made sure of the seal. No vacuum leak now. I took it out yesterday for another test run and I got a P0300 code PCM. So I tried to reset PCM by disconnecting battery and connecting +- cables for 30 min. During the warmup I got P0306 code, scanner says injector. I checked resistance on all injector leads and they all are 13 or 14.
I am at a loss! If anyone has any ideas, I would love to hear them. Thanks in advance!

Those spider assemblies were a mistake. I would do a running compression test and verify the mechanics or not before chasing other things.

I did a running compression test and all cylinders were good. Any other thoughts?

What were the readings?

What is the fuel pressure reading?

What were the idle and snap throttle pressures? Did they all fill similar?

Fuel pressure is 60

My scanner reads and graphs fuel trim, but I’m not a trained mechanic so I’m not sure how to interpret the fuel trims.

compression on all cylinders were right at 150

That'* a cranking test?

Yes. Started the engine and then shut it off on each cylinder.

the other thing is my scan tool does not read CMP retard. After doing more research, I think I need to have this set. The spec is +-2. I originally had the P1345 code, I repositioned the distributor and the code went away. But I have not had it set by scanning tool. I think this is my next step. I’m also going to recheck for vacuum leaks around cylinder 6 of the intake.

So engine is running. Your number of 150 is not right. That may be for cranking but for running it'll be a lot lower on pump and much higher on snap throttle. You did not do the test correctly.