/6 head

As far as I know, there is no MLS gasket for a slant six. The steel gasket I am talkining about is the OEM steel "shim" gasket. Single layer embosed steel.
I will grant you, if possible it is best to resurface both the head and the block, but How many times have you heard of the block being machined, when replaceing the head, and/or gasket (talking engine with steel head and steel block). Many times when a head is rebuilt (ei valve job) the deck surface also gets a light cleanup, but not always.
Clean the head and top of the block really good ( I don't like power driven scotch brite pads), and check the head and block for flatness using a straight edge, and feeler gauges.

For a sealer I used to use aluminum spray paint (in the old days), or Copper Coat. Now I like to use Hylomar