Nice frames! Yes, I think Shimano 600 (Ultegra)
will be perfect for the AB, both time- and colourwise.
Re. the A-frame, did you have a look at my Gazelle Champion Mondial site: jvs.webklik.nl
? Using the frame number (under the BB) you can determine the year of construction. There are some brochures/spec sheets from around '76 on the site as well.
Lots of possibilities for the A-frame. You could go for a Japanese mix of SunTour (VX or Cyclone) derailleurs, Sugino Mighty or Sakae RIngyo (SR) crankset and Dia Compe or Weinmann brakes. Italian is another possibility, with Campagnolo (Gran) Sport derailleurs, crankset and hubs and Universal brakes. Or perhaps a French build with Simplex derailleurs, Stronglight or Nervar crankset, Mafac centrepull brakes and Normandy hubs? Any of these choices would look good on that orange!
I like the sound of a herd of Gazelles... maybe I should change the thread title?