Sure wish you would have mentioned this about a week ago. I am pretty sure that is where your problem lies, not to mention there isn't a whole lot left to consider. If it started this immediately upon replacing the head, that is where I would have started to begin with. With this revelation I don't think a pressure cap will really do anything for you.
What do you mean "I had the head off to fix it" what did you do to it? I was assuming you bought this engine from someone, and just plugged it in. But once you remove something like this you stir up a lot of crud that could move around and plug things up. There are a lot of small holes in the head that could get plugged up and not work. I don't know all the variations, but it might be possible to have gotten a head gasket with holes in the wrong place or it is upside down blocking holes or something.
Whatever the case, since you have the front end off you might as well pull all the freeze plugs and clean this thing out, looks to me like you will be pulling the head anyway. Because as I said before, there is no percentage in overheating an engine over and over, no good can come from that.
