I just purchased replacement Dayco belts:
Alternator part# 15400 (smaller number on carton sleeve: 11A0900) I believe 15400 indicates 40" belt
A/C compressor part# 15355 (smaller number on sleeve: 11A1015) Indicates 35.5" belt
I, too, have a 1972 with original equipment brackets and components.
Since there is some adjustment on the alternator, I imagine desert_detomaso's numbers are 41" and the other number referenced is 40.5", and mine is 40".