I have a rebuilt 350cid in my 71', and when the rpm drops below like 1000 rpms when i stop or slow down, the head lights and guage lights dim and the tach trips out. Also, my pwr string pump squeeks loadely when i turn the while until i get going about 10 mph. So i am wondering if my bigger cam causes a lower idle speed than stock therefore under spinning my accessories. Should i get a bigger diameter main pulley? Any help is greatly appreciated.

First try the easy/cheap stuff.
Make sure you don't have any more than 1" of travel when you push on the belt between pulleys. If you have that or more, tighten the belt a bit. Check the belt for wear/cracking/melting, if found replace it. Make sure the belt seats properly in the pulleys, and make sure you have all the same pulleys and not standard/deep ones on the same belt.
That should help with the squealing on the one pulley. As for the dimming lights, are you still running a stock alternator with an external regulator? If so you might want to upgrade to a higher amperage alternator, with an internal regulator. Easy to install, and there's a tech reference to guide you along the way.

Fred

Thanks, I will check the belt travel, where is the tech refence that you referred to above? I am definitly thinking of replacing the alternator, but shouldn't i use a multi-meter to measure the amps being pulled from the alternator first?

The multi-meter route would be a good idea. See what volts are being pulled. But if it's anything like my '70, going to the 1-wire was almost a no-brainer.
Here are the Tech References. (http://www.chevelles.com/techref/)

Fred