If you want to install a Simms recoil pad you will have to have the stock flattened out and have a pad ground to fit the stock.

If you reread the fit guide it should tell you that it will fit a Remington BDL flat stock.

Simms "precision fit" recoil pads are far from precision fit. They get close but usually either leave wood exposed in spots around the pad or the pad is too large in spots.