Oil Pump Fitment

20160909_150936.jpg 20160909_150936.jpg Engine is a 1977 360. I bought the engine rebuilt and did not have an oil pump. I purchased a melling 72s pump.

The mounting neck will not fully seat, which does not allow the pump outlet base to mate up flush with the bearing cap.

There is a "step" (see attached pic) View attachment 1714966625 in the bushing hole that may be causing the problem. I don't have a mic or caliper that I can get in there to confirm dimensions against the pump.

Has anyone else run into this? What's the fix?