Go with your plan Dale.

The locking lug arrangement you describe is called a "shoe lump" and it's just fine. Many top-shelf rifles exist which have shoe lumps.

Your barrel machining idea has also been used, and will work great.

A double rifle (any double rifle) is not the best long-shotString rifle due to the barrels/ribs/etc joined together the way that they are -- but really, you'll be fine.

Cheers
Tinker