81 cylinder head vs 67 head

Well first off, the slant six received hydraulic lifters in 1981. So, that head is totally different than the 1980 and prior. The valve cover is even different and non interchangeable to the 1980 and prior. That said, there were a couple or so "revisions" of the slant six pre 1981 head. The best thing to do is read this:
Revised SL6 Combustion Chamber