The classic French derailleur hangers are the oddballs. At some point Campagnolo was using 10mmX26TPI while everyone else was using 10mmX1mm, but they are close enough not to be a problem (snug fit). I'm not sure when that was changed. Anyway, you should be fine with the derailleur hanger design and threads.
You should be able to calculate your brake reach by measuring from the dropouts to the brake mounting bolt, or perhaps if you used a string cord between the dropouts, add 5mm for the axle radius.
Your 700c wheels will have a BSD diameter of about 622, or radius of about 311mm (to the bead seat, lowest part of your brake shoes, a couple mm more to the middle of the shoes).
27" wheels will have a BSD of 630mm, or radius of 315mm (plus a few mm to the middle of the shoe).
I'm not sure where you find the specs on vintage brake calipers.