GM one wire alternators are famous for not producing much volts and amperage until around 1200RPM.
to start, check the alternator tag for total amperage. [i think it should have a tag stating that info]
then, check the voltage coming from the alternator, using the multimeter on the DC setting. red lead to the output stud, black lead to the case. it should read 13.8-14.2 [or very close to that].
next, check the voltage at the battery and compare that to right off the alternator.
please report back with what you come up with.