I just did this conversion, I went with a 10si. I ordered an alt for a '79 Monte Carlo with A/C.
Here's a link to that thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=820083
A 12si alternator wires in exactly the same.
A CS series alternator hooks up almost the same, except you have to install a resistor.
Probably way more helpful than my thread; here's a link to the Faq by VetteVet:
http://67-72chevytrucks.com/vboard/s...d.php?t=815300