Sadly Cal you may have to get right tough with it to get it out, I had to mine.
I actually ended up using a 1/2" socket extension and kept rocking it to and fro while giving it the occasional whack with a hammer to break the (you got it right), steel to alloy electrolytic / galvanic corrosion!
It`s a lovely little road salt trap down in there, the secret ingredient that starts the whole problem off.
My coolant stub was quite badly corroded and it was fooked completely by the time I got it out, luckily I found one in a breakers yard in good condition for little money (Yamaha wanted something like 26 quid from memory) and it had several coats of Smoothrite applied to it and oven baked to `hard as nails` before I installed it with a new O-ring and a liberal coating of water pump grease.
I used a new stainless cap screw to hold it in place, again the thread coated liberally in water pump grease, I don`t expect any further problems with it now.
is this one of those steel to alloy self welders 
Was the sealing surface on the head eaten away at all by the electrolysis?
