Interesting project, I've been following a similar path. I currently have a Ruger #1 being re-barreled to .45-2.6" for use in developing loads for a double in that chambering. My goal is to reproduce the .458win in a rimmed cartridge; the action I am using is very strong so it can handle the pressures. My measurements of the Starline cases indicate they should have similar durability to .45-70 cases. I should have the rifle in a week or two and will let you know how the brass holds up.

That being said, you should be careful about wildcatting a DR using shotgun frames. For what its worth, I was planning a conversion on an '80s Merkel 12ga and the math was not working out too well; using the proof mark data on the frame I was coming up with typical .45-70 capability. After talking to the head Merkel smith at the importer I abandoned the project; according to him, the shotgun frames of that era are not as heavily built as the DR frames.