Pcone3

hello everyone, I'm sort of new to the big block world and could use a little help figuring out where this knocking is coming from. I recently purchased a 1988 K3500 single rear wheel 4X4 with the 454. It did not run when I purchased it and had been sitting for a little over a year. The guy I bought it from said it had a bad knock coming from the engine and it was hard to keep idling. I put some new gas and seafoam in it (still had approximately 1/4 tank of fuel from when it was parked.) after a new battery and two turns of the key, it fired right over and was idling very erratically. It would surge up, then die down until it almost choked itself off, then surge back up again. I could hear the knock very plainly and it sounds to me like it's coming from the top end on the #4 cylinder. I removed the valve cover and noticed a large amount of carbon build up around the push rod. My first though is that it's a valve issue of some kind, but I'm not entirely sure. Also, could the idling issue be fixed with new plugs and wires or is the erratic idling part of the knock issue? Any advice and feedback would be welcome!