According to my NAPA Prolink electronic catalog, there were 4 possible alternators on that year/make/model:
2 that were 55-63 Amp "small frame - 10SI" ... (1 with single groove, 1 with double groove pulley)
There were also 2, 80 Amp "large-frame - 27SI" alternators ... again offered in single and double groove pulley versions. These alternators had a 2-leg lower mount and were an "HD option" on these cars. They were usually on the well-equipped models with AC, power seats, rear defrost, etc.
Most Delco alternators have a Delco-Remy ID number stamped into the front half of the case along with the Amp rating. It is also common practice for rebuilders to grind these numbers off of a "rebuildable core" and affix their own identification label or tag according to the rebuild specs.
IF it is still there, it will be all numeric ... something like "1101015" followed by "80A"
I can help to idenify it further if that stamping info is still available, or you can browse the
Note that the Wilson catalog only list 1 of each series, because their alternators are not supplied with new pullies.