well my old 96 200k plus silcerado 5.7 2wd 5speed has a stumble at speed .i put in a new motor a 1000 miles ago .idle smooth power good and smooth jut at times it has a hiccup just for a moment its a very noticeable will do this once or twice then that’s it with the new motor i did new plugs stock ones yes i gaped them wires to cap ans rotor .no trouble cods used a snapon scan tool
maybe you can rephrase this question when you are less frustrated and more sober.
I hate to say throw parts, but maybe the old ignition coil(s) is(are) tired?
Why did you put in a new engine? This only had the problem after the new engine? Is the new engine under warranty?