continued

Backfiring is almost always electrical, unless something serious like a bent/stuck valve has occurred. Some Chevies have a problem with wiped camshafts. Where is the backfire? If it's in the intake, that indicates either crossed wires or poorly seating intake. The decrease in idle stability indicates a possible intake valve problem.

I think a more extensive investigation is in order. The fact that pulling plug wires from the cap has no effect on one bank is a serious tell. Mechanical problems are much more rare than simple electrical stuff, therefor I still would check the firing order first. Perhaps a compression check on the dead bank is in order while determining the true firing order.

It's very easy to damage a camshaft during break-in. The timing chain could be a tooth off. All speculation.

I'd check the firing order by checking compression strokes. Perhaps pulling the valve cover on the dead side may help. It will show the true firing order, and any bent/stuck/improperly opening valves will be apparent by looking at the actuation of the valves, the height they sit at, and the amount they open.

Bill