Alternator

You listed two different cars. I'm guessing you have a Caprice because a 350 c.I. Would flatten a Cavalier! Check rockauto. Com for the choices if you can't find a generator locally. They list a 100 amp, 105 amp, and a 120 amp unit. You might find the rating stamped on the side of the case. If there's no rating, go with the 100 or 105 amp generator. That is WAY more than enough for any car. If the car came with a smaller generator and you install the larger version, you run the risk of blowing a fuse or fuse link wire when someone performs a load test on the system. Other than that, there aren't enough things on the car to draw a total of 100 amps.

One word of warning; there's no polite way to say it; GM went from the second best generator design to the world's worst pile ever beginning with the '87 model year. Due to their design, they develop huge voltage spikes that interfere with computer sensor signals, and they can destroy the diodes and voltage regulator inside the generator. It is real common to go through four to six of them in the life of the car. What many professionals are finding out is to reduce the number of repeat failures, replace the perfectly good battery at the same time. As they age, they lose their ability to absorb and dampen those voltage spikes. The old battery will work fine in a '86 or older car.

After looking again, it appears the mounting ears and holes are different for the different current ratings. You should be able to identify the proper generator you need by the pictures.