I can't respond to the pistol grip, but, I have the forend.
I bought mine YEARS ago. I also have an older Choate pistol grip stock.
Anyway. . . . .(I digress, terribly). . .
On my forend, I had to trim the forend where the barrel groove is, on the 'receiver' end. The groove or channel is just not wide enough back there. (See your photo, where the metal tube is exposed. There is the area (rubber coompound) that needs to be opend up.)
What I did - - - chuck a sanding drum in the drillpress that was a little larger in diameter than the barrel diameter, toward the chamber.
CAREFULLY open up the groove on the back of the forend until the action will open and close smoothly. (I created a 'taper' in the groove of the forend.)
Lastly, being curious, have these forends fallen out of favor?
I really liked mine when it was on my old 870. In fact, when I do my next 870 for HD, this forend is going back on!
Keith