The bulge is intentional. Flat nosed 45/70s won’t make the corner into the magazine without it.
ETA. I see you figured that out.
What is the gain in going to the larger tenon? I can’t see the .007” per side making a huge difference strength wise. IME the chamber will bulge and the receiver will crack through the front two scope base screws and the thin web between the barrel threads and the magazine tube hole if you blow one up. They’re so thin through the front of the receiver that those holes all line up like the perforations in a check book and split them longways. I’ve seen, handled, and inspected more than a couple that had let go in this fashion and none ruptured the barrel.
I'm guessing the strength difference would be so minimal there might as well not be any?
Every one of these I've done has the "facing" part of the 1st reamer making "first" contact at a different position on the receiver face.
The cutting/rethreading effort step is done as an effort to align the bolt,barrel,bore axis , the way it does when you "true"(not a term I like) a bolt action receiver,at least in theory.
Also,installing a little bit better quality barrel cant hurt anything while on the way to changing calibers.
From your post I'm pretty sure you got that and the explanation was for those that didnt.
I'm in agreement with the holes for the scope being a strength issue and often wondered why there wasnt a dovetail of sorts running along the top of the receiver (like CZ) to attach the rings to and add a little rigidity.
The bulge in the barrel had me pretty steamed til I figured out what was going on, then I was so embarrassed/pissed at myself I had to stop for the day. Oh well ,yet another teachable moment applied and my "Super Smith" status is forever diminished.