Evan,
FWIW, and not to start cross-fire
, I decide to check my factory service manual for my 1977. All are shown as having 1.28 inch brakes, except the hydroboost (hydraulic) which had 1.53 inch.
If you have the system on the glove box door, the book shows the JB1, 3, 5, 6, 7 all have the 1 1/4, while the hydroboost numbers are JB8 and JF9.
If you had to make a purchase cold, it seems like you'd be more likely to use the 1 1/4 rotors. I realize there's a slight difference in years between yours and mine, but the odds would seem to favor the 1.25's.
Juzt my dos centavos
Brian