IMHO there is no earthly reason to use lead/tin solder on any double, even for laying the ribs. the new solders containing 95% tin + 5% silver appear to be far superior, much stronger, melt about the same (a little hotter, but not much), and I'm led to believe the barrel-group can be caustic blued if the need ever arises. I think Brownells market this stuff as 'Force 44' in the US. Any resident gunsmiths have an opinion on this?

I agree with 4seventy on the monobloc, though 5%silver/95%tin would probably be fine for fixing threaded barrels in place.