Can't find misfire

If the head is still on, retest compression in #6 dry, then put about a teaspoonful of oil in #6 and retest and see if the compression level comes up. That may tell you something about the ring condition in that cylinder; if the compression rings are worn, the compression should go up with the oil added.

You should reuse the later head; the casting is better; you can read up on that in the 'Articles' on www.slantsix.org. You can find the proper procedure for manifold tightening in another article there.