There are several possibilities.
Assuming that your measurements are correct, i.e., your "rebuilt" 60 amp alternator is really putting out 100 amps @ 1,400 engine RPM
1. during the rebuild
, either the rotor was rewound with more turns than the original, OR another rotor with more windings was substituted;
2. the alternator is somehow getting full-field current
, without regulation;
3. the alternator has an internal regulator
which is connected and is not operating properly;
4. the external regulator (MC-612) isn't connected properly or isn't working properly.
These last three are unlikely in and of themselves to explain a jump from 60A output to 100A output, but could be part of the problem -- together with the rotor-with-more-windings possibility.