vincenth
Retro Guru
I am a little bit flexible, the overall look and feel of the bike should still be retro. For example I wont be using v brakes but modern parts like tires, maybe seatpost or bottom bracket are fine.If you're 'flexible' about period correctness a later Dura Ace or Ultegra rear mech can be quite light. 7800(185g), 7700(195g), 6600(205g) for example and they should be fine for up to 28 teeth.
I recently build a 3x mtb with a short cage and srp pivot bolt, might do the same for this one.