Well, that's been a long time ago, but I'll try.

Clue me in, does your 70 have sequential turn signals? I don't remember.
Do the front turn signals work  (do they come on when selected?).
 If the front signals light but the rear sequentials don't work, the problem is
likely in the sequential mechanism. There is a chance that the Turn signal switch is
faulty due to excessive current to the sequential relay pack, but the most likely is a
concern in the unit itself.

 I fyou don't have sequential TSs, then test the turn signal switch, assuming the
front sigs light.

 OBTW, your "really big car with a really big problem" is also a really big want in
my world. I'll gladly take it off your hands so you don't have to deal with the
electrical problems :) I wouldn't want you to be saddled with such a mechanical
nightmare ;)
         Enjoy your Bird, Tom Adkins