More to the "what's your point" argument - If you want to restore a vintage bike for an occasional "show" ride, OK, but then obviously, you need to stick as closely as possible to original equipment.
If your intent is to build a tandem to actually do any serious riding on, sell the Gitane and buy a more current ride. Almost any used, quality tandem in the $500 - $1000 range will give you a ride that is head and shoulders above the Gitane. You'll spend at least that much just trying to make that one rideable, and you'll still have a very noodley, standard steel frame.
__________________
BICYCLE - [[I]bahy-si-kuhl] - Noun :> A medical device used to correct the common geriatric condition of OFS, (Old, Fat & Slow), in a manner that does not induce brain-decaying boredom like walking or running.
2005 Trek T2000 tandem, Giant TCR, Eddie Merckx Majestic Ti, Fuji Team, Giant Revel 29er, Windsor Clockwork (Orange) fixie, and a BikTrix Juggernaut Ultra 1000 fat tire eBike