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I don't believe DA Grizzlies are threaded for chokes, at least my 1st shipment one isn't. You will have no issues with buttstocks, forends will need fitting (except Surefire forends, they fit fine).
The threaded DA barrels do not use Remchokes. They're threaded for Winchoke, Accu-choke (Mossberg) and Invector. All 3 of those will fit. Yes, 870 parts are compatible. Like I said, the forends require slight fitting.