Okay, I can probably figure this out but it's defiantly easier for me just to ask.

It's a 1987 IROC, I just stuffed in a big block which means I'm running a different alternator; question becomes should I just change to the old alternator or rewire the new one?

The old alternator "clip" (the clip that clicks into the alternator, not power and ground) prong doesn't fit into the new alternator. (New alternator on the big block is an older style). I will refer to the factory small block alternator as "A" and the new big block alternator as "B".

"A"s wiring harness has 3 wires running to the clip/plug and "B" only requires 2 wires to run to the alternator. Why is their an extra wire, any idea what it runs to? I'd swap the alternator but "A" is a serpentine belt setup and "B" isn't and by the looks of it the pulley on "B" won't fit on "A" and I'm not even sure if they mount up the same.