That must be because the RWD SR20DE uses a slightly thicker oil pump gear and oil pump drive collar. Possibly even the same one as the RWD SR20DET. If you could find the oil pump drive collar from a FWD SR20DE you could probably use that in conjunction with the G-spec spacer and DE pulley to get the correct alignment.
Or if you do have to machine something, maybe machine the spacer instead of the pulley? Just a thought.
Or if you do have to machine something, maybe machine the spacer instead of the pulley? Just a thought.