Yep, good question - the bolt is actually 29 mm, I tried pulling it this weekend myself but my socket set goes 26-27-28-30-32. So I guess I can't tell you for sure, but that's the only bolt I saw on my '90 resembling a block drain. (I'm changing the head gaskets on mine too) Let us know what you find out!
As for the head bolts, all I have is the Haynes manual, so we're looking at the same info. Good luck!