I brought the new springs back to Stengel Bros in Allentown, PA and they de-arched them an inch. We'll see how it goes when they go back in the car next week.
Pulled the cover off the 10-bolt rear differential. Looks like it has been rebuilt recently (or at least prior to the last couple decades that it was put into storage). New clutches in the posi and the original 11/1971 dated 3.42 gears are thankfully, still there. Factory "CM" code stamping on the front of the passenger side tube was intact as well, dated 351st day of 1971. That's three weeks prior to the January 11, 1972 ship date.
BTW, here is the spec page for the part #80-335 1970-73 T/A and Z/28 leaf springs:
http://www.stengelbros.net/80-335_p_2927.html