I'm surprised that 70s/80s frames only have a single bottle cage mount. A 25+ mile ride for me absolutely requires two water bottles.
Do you guys either not ride your vintage frames on long rides, use camelback hydration, or use clamp-ons like these?
VO Bottle Cage Clamp
Of course if I'm willing to repaint a frame, I can get a second set brazed on. One of my favorite riding bikes (a ~1983 Trek 613 frame with a modern drivetrain and STI shifters) has been modified with a second set of braze-ons. A lovely riding bike that I prefer to my all-plastic-wonder 2011 Cannondale Synapse. The only time I pull out the latter is when I need every last bit of extra stiffness for a challenge ride or a group ride (where I'm often on the verge of being dropped).