Were I you, I would not worry about getting that plug out. You can adequately clean the head without doing so, and given the bivalent corrosion between the iron and the alloy, the chances are if you do get it out you'll do some serious damage on the way to doing so.
The problem with Alpine engines overheating is not with the head anyway. The problem is that the coolant flows too slowly at the back of the block and the slow moving coolant let's junk settle out in the jacket, effectively silting the block up and developing a thick, hard cake in the jacket. Remove the block's freeze plugs and thoroughly clean it out... particularly the ones at the very back of the block... very likely you'll find an inch or more of crud back there.
Of course, the other reason they overheat and we see damage to the #4 chambers so often is that on later heads instead of taking the hot water from the absolute back of the head (where the old cooling outlet was) they take it from about 1/2 way in the middle of the head, and this leaves kind of a 'dead' area at the back of the head where the water doesn't really 'flow'. The only cure I know of is to either install a head with that rear coolant port, or drill your head and install one.
Another reason they don't flow well is that very often you'll find pieces of casting flash in the cooling jacket. I found one in mine, and it was a solid lump about 1/3 the size of my palm, and 3/8" thick!
