Look what the UPS man brought today! Awesome help by DDMRK who provided the lower steering column and center duct for the A/C system. I can't thank you enough. That's what makes posting all this insane saga worthwhile - meeting fellow enthusiasts who will go out of their way to help a fellow crazy person in need.
And here it is installed. I compared the ducts to the ones on my Grandma's 75 Firebird that she bought new and we still have. The only difference I could see was that there were two mounting screws holding the column duct to the center duct on the ducts from DDMRK and one mounting screw on Grandma's '75. I also noticed that the 75's ducts had some nice sloppily applied black duct tape at the joints, so I sloppily applied it on my car, too - a family tradition, I guess!
Oh, and before I forget, a couple people have asked for a shot of the 1972 shaker hood switch and bracket. You can see it here, attached to the gas pedal. Both the four-speed and automatic cars used the Turbo 400 kickdown switch to activate the shaker. On the automatic cars, the switch operates both the trans kickdown and the shaker; on the stick cars it only activates the shaker solenoids. The pink wire goes to the fuse box and the orange wire extends through a hole in the firewall and has a female receptacle that mates with the male end (OK, we're getting a bit R-rated here) in the bottom of the shaker unit, thus completing the circuit when you stomp the gas pedal. This eliminated the bracketry, wiring and plunger switch mounted on the carb in '70 and '71.