pipe adapter for shower drain?

HELP! My new shower in the basement is exactly the same as the old one except for one thing. The drain assembly needs to hook up to a 2 inch inside, 2 and 3 eigths outside diameter pipe (black PVC 2") The drain pipe in the floor (concrete) is 1 and 3 Quarters inside and only 2 and an eigth outside! The shower comes with a special seal that fits over the pipe and thus it needs to be the right size. So....is it possible to get a slightly different sized seal? Or how do I go from this old metal pipe to the newer PVC type of the right size?