Lifting the engine a few inches did the trick, I was able to get a good angle that allowed hex head to seat deep enough. It looks like a previous owner possibly rounded out the tip of the bolt head, so I really had to pound the hex head in and thankfully was able to break it free without rounding it out further. I'll definitely use a new bolt when putting it back on.

So now that the housing is off, what else should I do while I wait for the gasket to arrive? I.e. what other jobs are easier to do with the housing out of the way. I was thinking drip timing the IP might be easier.