"The carbon on that valve is crazy thick! I've never seen anything that bad."
Think I found the culprit...after the issues with 2 of the lifters, I decided to pull the camshaft to inspect....
It has 2 issues - the first is this corrosion on the #2exhaust lobe that corresponds to the lifter with the corrosion on the base....
It could probably be polished up a a bit, but the second issue is the deal killer....lobe for the #3 cylinder is flat...I would guess this corresponds to the issues with the valve seats and extreme carbon build-up on the #3 valves....maybe its a bit hard to see in the photo, but the last of the 4 lobes in the pic is the flat one - very easy to see and feel in person.
So new cam and lifters it is....The current cam is stamped GM, so it appears to be a regular factory non-performance cam, and I guess I should be ok to get a standard stock replacement with new lifters and not cause any issues with pushrod length or rocker arms etc....
I've got some other concerns as well as two of the pistons may have an issue - not sure if something with the rod bearings or maybe a ring issue....guess its time to flip the engine over and look from the bottom....