It occurs to me that small blocks Chevies had a problem with worn cam lobes that cause a loss of power and backfiring under rapid acceleration and the 4.3 is basically a 5.7 with 2 cylinders eliminated.
Your symptoms kind of remind me of a chevy pickup I had worked on about 17 years ago with TBI. Idled fine, wide open throttle fine. Had a tip in hesitation with light throttle. The fix for that one was to remove the throttle body and clean passage for MAP sensor.
To check for a stretched timing chain, remove the distributor cap.
While someone turns the engine over by hand in it’s normal direction by the crank bolt watch the rotor in the distributor. Now have them turn the engine over in the opposite direction.
If the crank can be rotated more than a few degrees in the opposite direction before the rotor in the distributor begins to rotate the timing chain is stretched.
Tester
So if things are mechanically sound then why the blobs of crap on 2 comparatively new spark plugs?
Spark plugs tell the tale of the condition of the combustion. If you have ignition problems, all kinds of odd deposits can form on the plugs. Old school mechanics can read the spark plugs and diagnose all kinds of problems. If you can post a picture of the plugs, I’m sure a few of us can give you a better explanation.
My thought is you are going to find the trouble with either @TwinTurbo s idea of the EGR valve or with @Tester with the timing chain issue.