The question of the monobloc "step" being a possible weak spot has been a puzzle for me too. Sure it has an angle, on the other hand it has been succesfully used in double rifles. The most notable example that I can think of is the Beretta Sable OU. There appears to be no radiusing at the "step" in the Sable.

Another puzzle is the proportion of metal thickness apportioned between the barrel tube and the monobloc. I have seen (from the lines under the extractor, that the monobloc is often relegated to the thinnest possible section, acting more like a holding fixture, leaving the pressure containment to the barrel tube. Is this the best option?

As to the pictured project, I LIKE IT! A 6.5mm double must be a pleasure to hunt with.