Well, I've been reading through all of these comments and I think I'll interject my $0.02, too. When I decided to respread my travel bike, a 1980 Trek Columbus SL short, small rear triangle frameset onto which I had installed the S&S coupler system, I had the 120mm spread out to 126. When I decided that I didn't like the time required to remount the rear derailleur, etc, turning a hotel room into a bike shop, I rethought the process and respead the frame to 135mm in order to mount the Rohloff 14 speed hub system. No problem with the frame. You or your mechanic have to be a good one, cognizant of holding the bridges rigidly so as to not split (for every action there's an equal and opposite...) anything about these areas. A basic rule for bending steels is that things get dicey when the radius of the bend approaches six times the radius of the material being bent. (smaller than 6 is not good)
I would also say that I have had a '50s Follis's rear triangle with pencil-like seatstays bent more than the above-mentioned Trek just in order to bring it back into alignment before being restored to its original candy apple red glory. Still looking for a few original-era components there.
However, when I acquired a never-been-built '72 LeJeune last year, I decided to use a mixture of period-correct stuff (Stronglight 99 double) with newer gearing (Ultegra 9) and Berthoud fenders so as to afford me the luxuries of my exact riding positions (all positions, even the same ergo bars I'm used to). I consider it a massive success and thoroughly enjoy this bike, but some aspects are probably a sacrilege to purists. That was my first foray into Sheldon's idea of "old bike, new parts". I would never consider doing anything like that to the old Frejus track or the '61 Legnano. Even the 82 Vitus.
Ken Wehrenberg, Hermann, MO where it's very hot today