Thanks for the suggestions.
I got it off with a pair of visegrips and put the death grip on it. Nice teeth marks in it now. I did try wrapping it in a piece of leather and some rubber, but it kept slipping marking up the cap anyway. Oh well.
I got all the brass soaking in white vinegar for the night.
I got chunks of crap in the gas tank. Besides filling it up with some BBs and shaking it it around loosening up the "chunks of crap", anyone find an alternative method of cleaning the inside of the tank? I just don't have the BBs at this time!