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Haven't read every word of this thread but has anyone mentioned the Paramount tandem and the Town and Country? They were fillet brazed. Fun fact, on the T&C the fillets were covered in lead solder (!) and the lead was then smoothed, apparently cheaper than smoothing the brass fillets (more commonly called bronze in England). I also have a Twinn tandem, a cheaper model, with most of the joints fillet-brazed, though not particularly well! It has held together for over 50 years though, so it's good enough.

Also the Excelsior (mine was from the '30s) was fillet brazed, as presumably most any Schwinn made before their uniqure welding method was invented.

I put a Campy headset in my Schwinn Twinn, partly for laffs but also I was replacing the fork and the new fork had a Campy-sized crown race seat, so I was going to have to shim something somewhere. My fix was to slide an entire Reynolds 531 head tube down inside the Twinn head tube — perfect fit! I could have sliced off two rings from the 531 tube but this way proper alignment between top and bottom was assured, and one less hacksaw cut! It's not as if we worry about the additional weight — the bike weighs 80 lb and I'm not joking, that's actual. It also allowed me to solve two other problems, namely the frame size is too small and I wanted the bars up higher, and the fork I was using had a long steerer without enough threads to fit the frame as it was. So I just left the 531 head tube long by a few cm. Brazed it to the top and bottom of the Twinn head tube to immobilize it, and sprayed the braze area with some rattle-can paint et voilą, a Twinn that's actually rideable and quite useful. We use it for grocery shopping, going out to dinner or to friends' houses, picnics in the park etc. The new fork I put on is tubular (as all good forks are — sadly not the Twinn whose fork was borderline un-rideable) and it has cantilever studs, so we have one good brake now. The original brake was a wimpy aluminum sidepull long enough to reach around a balloon tire. More like a "brake-shaped object" than an actual brake, plus the rear is a worn-out Atom drum brake, maybe even weaker than the front sidepull. Now we can ride down hills without uncontrollably gaining speed all the way down!

BTW for anyone wanting a more modern headset in an old Schwinn, you don't need a 531 head tube, that's just what I had handy. Any 1-1/4" tube can work, though if it's too thick it'll need more reaming.. Best is to get a piece of Cr-Mo, which comes in .035" wall, which is perfect for Campy and most any other normal (not Schwinn or BMX) headset. Cut two rings if you care about weight, or just one long piece as I did. Brazing isn't necessary, you can fix it in place with Loctite. They make special formulations made for mounting loose-fitting cylindrical parts like bearings, and the stuff is ungodly strong. I forget the formula number but someone here will remember...

I don't know anything about "modern" Campy brakes but you can definitely use old Record, the ones that came out in the '60s. If a Drop-bolt (made by Campy or certain after-market versions) won't reach, you can make a drop plate, a simple fabrication job that most any home handyman can make if you can hacksaw and drill a couple pieces of steel. I'm sure they've been discussed here but anyone can ask me for instructions if you don't know what I mean.