[ChevyTech]

You have a steady fuel pressure of 12.5 so look for a problem else where.

As far as the pressure dropping as soon as the pump stops – this is OK.
Some TBI units had a hole in them that bleeds off the pressure as soon as the pump stops, and I have never been able to determine which TBI unit had the holes, without taking them apart.

The IAC controls the IDLE speed and will not cause a stumble when the throttle is open.

The electronic spark control can cause a flat spot or feeling of lack of power. The Knock sensor contains a piezoelectric crystal which causes a voltage to be produced. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system retards the timing because it determines it is pinging.

The backfiring makes me think the engine may have a mechanical problem.
The head gaskets do fail on these engines and when the head gasket begins leaking between the center cylinders on either bank it can cause what you describe. These engine often ping when the head gaskets are beginning to fail.

An exhaust lobe on the cam that is getting flat can cause a backfire. When this is happening it is most noticeable when you crack open the throttle hard, when the truck is in park, with the air cleaner off. The ZDDP change in motors oil seems to have made this a more common problem.

Don't overlook an easy fix like a bad distributor cap, with a carbon track, leaking spark to the wrong cylinder, causing the problem.