Well, you've got an easy open frame and with split locking lugs. I don't see where there should be a problem but obviously there is. If the frame was stretched then you'd have the gap regardless of barrel. Take a mic and check the distance from the pivot pin to the front of the frame on both for comparison. There are others out there more knowledgeable regarding Contenders than me, Ladobe is the first that comes to mind. Maybe drop him a pm, see what he has to say.
As to the flat/stepped base, the metal piece that's welded under the barrel that the barrel pivot pin goes in, the earliest ones were flat where it would meet the frame. Later ones have a slight step milled in.