Dave,
If I were going to do that, that is the way I would level it. You should be careful, though, where you place the bases. The rib, itself, is likely soft soldered to the barrel, and I would not trust it to stand up to the recoil. The rib extension, over the chambers is likely either solid( if an actual monoblock) or hard soldered(brazed) to the barrel ( if they are bundled), and would likely be OK. A close inspection should show where the joint is. Chose the scope to fit the spacing of the bases. Good luck, let us see how it works out.
Mike
