i strongly doubt that they would.
the R3 recoil pads from synthetic stocked Remmy's dont just fit the butt of the gun but actually have a flange that extends inside the hollow buttstock.
the Grizzly factory stock is both a different shape and an entirely different design - especially the hollow part of the buttplate - than the Remington so i dont think it would fit. i could be wrong - i never actually tried, just going based on what the inside of the Grizzly buttstock looked like. i sold my factory Grizzly buttstock and replaced it with an 870P wood set.
your best bet would be just to order an entire aftermarket stock for the Grizzly - either a take-off Remmy synthetic or a hogue or something - and sell your original Grizzly synthetic on the EE. probably wouldnt cost you much more than the recoil pad itself that way, and you wouldnt run into any fitting issues.
also - the forend tube on the Grizzly is slightly oversized, so youll have to get a broomhandle and a sheet of sandpaper and sand a thou off the inside of any 870-spec forends you intend to replace it with. if you dont, and its a synthetic forend, youll never get it off. if its wood, itll probably split right down the middle.