My buddy's 67 camaro is having electrical issues. It's a new 3-wire internally regulated alt but doesn't charge at idle. Does fine if traveling or if you keep the engine reved. Any ideas? Could it possibly be the pulley size effecting it?

No, more than likely wired wrong

What gauge are you using, to see a "no charge" at idle? The factory gauges are not very accurate.

They are aftermarket 'Shark' gauges.

It's wired as follows:

Alt to horn relay
Relay to battery
Battery back to Relay

Then from relay there is wires going to instrument panel, hide-a-way headlight doors, and electric fan.

if this is a stock alt for that year and is externaly regulated this is normal. It charges very little at idle and the headlights will dim at night at idle. GM's early alt was not up to par and is nothing like what you have on your newer cars and trucks. it can be corrected go to www.madelectrical.com he kas a kit and some instructions.
http://www.madelectrical.com/electricaltech/chevymain1.shtml