My current AC Delco alternator doesn't charge unless the car is running far above idle, hence being in town it never really charges. I want to upgrade or just plain switch it out but I do not know if this will fix the problem. Help please. Thanks.

If it were me, and I am going to at some point myself, I'd upgrade it to as high an amp rating as I could find.
Make sure to upgrade the wiring too if you do go bigger.
Your stock unit may be as much as 100 amps at whatever RPM, but idle current is menial.
The electrical demands being as high as they are with these old cars, it would only make sense to have more power than normally available.
Especially if you were to want to add any kind of power amplification, or air compressor or extra lighting even...
Stock headlights in our cars use tons more current than your garden-variety modern vehicle headlamps do to operate.
my .02

Your 69 caddy has a regulator on the firewall , not inside the alternator...Your problem couuld be a bad alt. or a bad regulator or it could be a bad connection...if you upgrade to a alt with a reg. inside, you will have to do some rewiring...

Well I got the new 63 amp alt on last night with all new belts and drove it for 10 miles and all seems well so far. We'll see. The new alt. supposedly has an internal regulator so who knows. Thanks for the good advice, and 63 is the lowest but it was all that was available. At least now the lights don't dim to nothing sitting at a light.