Ping / intake backfire on 350

Just wanted to gather some opinions on a recent issue with my '95 Yukon.

Background:
Last weekend I replaced the exhaust from the manifolds all the way back. new stock cat and Napa soundmaster muffler. (As opposed to no cat and a flowmaster).

At that same time I replaced the plugs, going from Bosh Platinum Plus (HR9BP) to Delco CR43TS gapped at 35.

(If you are wondering why I downgraded everything, truck was a little too loud for my daughter, and the plugs were $5 a plug cheaper..)

Issue:
Ever since, my car has run a little rough, almost like a miss. Fairly smooth in park at idle. At idle in gear is rough like it's missing on a cylinder, but never stalls. Rough running continues up past 40, but isn't so noticable beyond that.

Every now and then at low speeds, or when laying off the gas around turns I get a pinging/backfire throughthe intake. happens once or twice, doesn't do it again for 5-10 minutes. Doesn't seem to prefer cold/warm.

Question:
Could these Delco plugs be causing the issue, or could it be the recent addition of a cat and more restirictive muffler? Is 35 a decent gap? IAC is a month old, all the plug wires appear snug, no vaccuum lines appear cracked/unhooked. Curious if I should pend the extra $40 and pop in another set of platinums, or if the issue is cat/muffler addition related.

If you feel a definitive miss at idle, started only after doing these changes, I would focus on a possible bad wire, or even a cracked plug, recheck what you did.

Re checked them a couple of times, even swapped 6 of the 8 platinums back in. (lost 2) Still present. It's not so much a definitive "miss" as pulling a plug wire, but feels similar.

I could see it possibly being wires, improper gap @ 35, or maybe even timing. I'll be checking those tomorrow.

almost every time i change plugs at least one wire breaks on me it seems. i agree prob a wire or 2