missing at idle

Hey guys great site lot of good information first time posting. I've got a 2000 mile 350 chevy 60 over flat top pistons double hump heads new performer eps intake new edlebrock 650 thunder series carb new comp 268h cam I put this in my coupe that I bought and never could get it fine tuned and get a miss out of it come to find out the guy I bought the car from put a sealer in the gas tank and it never did dry and came thru the carb intake heads (black sticky tar looking stuff) anyhow I tore it down to the short block to have everything cleaned while looking at the heads #5 exhaust valve was white (lean?) took the heads to the shop my kid gets his race motors they cleaned them gave them a complete valve job I put in a new gas tank had everything else cleaned and put it back together with new plugs wires and cap I can fine tune it some what but still have the same miss at idle pulled the plugs and #5 is white all the other plugs are a tanish color my question is, is the exhaust valve I mentioned above sticking and not closing, a vacum leak, flat lobe on cam, or ? Thanks for some information in advance and like I said before GREAT SITE

PS. the motor guy that rebuilt the heads is out of town till next week