Long term update: The alternator listed here and the one that fits the 1982 Caprice use a 10.414mm (0.41in) wide belt, and my Z motor uses a 13mm belt system. The actual belt that zKars used is a 13mm (1/2") belt so the original instructions are correct but this cross reference section is not. Initially the 1982 Caprice alternator bolted right in and with a thinner belt it seemed to work OK. Under heavy load, perhaps due to my 3.0 Rebello motor, the thin belt gets thrown. I was able to source a replacement pulley and am happy to report that with the correct pulley everything appears to be working correctly. I attempted to find a better cross reference with a 10SI alternator that came with the correct pulley but I never found an OEM equivalent, though I did find several advertised for tractors that looked right. Here is the pulley info:
Delco Alternator Generator Belt Pulley 3/8 7/16 1/2 in Wide Single 1 Groove 10SI