rotor specs?

Original and minimum specs are from Chilton's "American Motors 75-86" (part #7199) for those years. 71-74 is from the 74 TSM. I double checked a 70 TSM to confirm tomslik's figures.

Model                                       Brake type    Original    Minimum

65-70 all                         Bendix four piston   0.500"      0.450"

71-74 all                               Kelsey-Hayes    1.000"      0.940"

75-76 (all) and 77-78 Matador  Big Bendix     1.190"      1.120"

77-81 (all exc. 77-78 Mat)        Bendix          0.088"*    0.810"*

82-83 all                                   Delco            0.088"*    0.810"*

*77-78 use a big bearing rotor, same bearings as Big Bendix. 79-83 use the same rotor regardless of brake type. All except 75-78 use the smaller bearings, same as used with drum brakes.

Yes - smaller diameter rotor. The small car six and V-8 used the same 10.8" diameter rotor. I couldn't find the specs for the 77-81 four cylinder Gremlin/Spirit, and don't have my TSMs for those years handy. I believe they were 10",  or close to that. The 2.5L GM four used the same rotors as the six and V-8, IIRC. Someone with a TSM can confirm the diameter. The caliper is the same, bracket changed.

I thought the 79-81 GM 2.5L used the same rotor as the six and V-8, but I'm apparently mistaken. American Parts Depot lists a different part number. They are the only AMC vendor that lists the 77-78 four rotor -- at $169 each (they use the big bearing). 79-81 are $59 each, $10 less than the six cylinder rotor (and a different part number).