I just bought one as well and it sounds exactly like the gun you purchsed!!
From what I have read and been told, most everything that is made for the Remington 870 will fit the PP except for the barrels and magazine extension tubes. So your choices are pretty much limitless. The barrel issue is a problem. I just put up a post the other day that I said that I called H&R's customer service and they informed me that they did not have any 18.5" barrels in stock and to call back mid-summer! I am thinking of chopping off my barrel as well so hopefully someone who has done it will have some insight. If not, I work at a manufacturing plant with a machnine shop so I can go and ask one of these guys what to do.
Good luck with your project. Keep posting your results and so will I. I have a feeling that we are not the only ones doing this.