340 smoking bad

If the idle is not rougher than it used to be, I'd guess valve seals like you said. Test for vacuum leaks on the outside of the intake manifold and carb with propane - does the idle get smoother? If so, then you have a vacuum leak for sure. It could be leaking on the inside too (like in the lifter valley area). Im not sure if there is a good test for that. I might try placing some propane down the oil filler, but then it could also go in thru the valve seals too.