Backfire through carb on light throttle (not carb related)?

I have been trying to source a weird light throttle issue my 70 elky has been having.

Engine details:
350CI GM crate engine (vortec "newer" style)
edelbrock heads & power package
holly 4 barrel carb

On very light throttle (fresh from stop or rolling through a parking lot, heck even at speed if just getting it some juice to maintain) the engine will stumble and backfire through the carb. Initially I thought this was a carb problem and rebuilt the original 4 barrel, no luck. Tried a newer aftermarket carb off my buddies shelf, identical problem. Idles fine without choking, warm up cycles as it should, electronic choke functioning.

What I've checked thus far:
Chem tool vac leak check
points & distributor replaced
timing checked & rechecked
plug gaps, wires, cap, all checked
found 1 valve was overtightened when checking the valves, corrected
new choke, and carb idle A:F tuned

Ive noticed if I richen up the idle A:F it helps a *tad* when the engine isn't as warm, though still bad enough the car will can choke out completely. From a stop, if you give heavy throttle off the line it will pull through the stumble and is drivable, engine has great power at 1/4 to wide open throttle through the RPM's. That said, I noticed if you give it a foot full through the gears and come to a stop, the next roll on has a bit more backfiring or stumbling then before (possibly heat related? I was thinking maybe more wind cooling everything off could be causing something?)

Where do I look next? Can I still have a valve issue? Bent rod (though I would figure this would make noise)? Insight greatly appreciated!