You might be able to loosen that valve by removing the rocker arm and wacking the valve stem a few times with a rubber mallet. You'll want to check the push rods too, don't think the lifters can absorb the entire lift of the cam lobe, so something's gotta give in the valve train.

 

I'll give the rubber mallet a shot.
I'm all too aware of the valve train issues this could cause.

Hoping it's more of a sticking situation due to sludge, as opposed to a seized valve.
There was no definitive "event" that one would expect if the piston strikes a valve, such as thud, ping, blam! etc. Just stalled quietly while coasting and started running very rough on start up. It does run much better after plenum and tune, but obviously still pretty rough.
There has been some exhaust backfiring so it's possible fuel is escaping out of the exhaust valve on down stroke here and there.