I have re-drilled the 336 base to fit a Benelli, but I don't know about an 870. Best thing to do is use a base from the 870 and set on top the 336 base to see if there is enough material to drill an additional hole. Hopefully you can use an existing hole in the 336 base and only have to drill 1 more. If you have to drill 2 holes it's a lot more difficult to do.