Or, is there usually a decent voltage drop when all of the lights are on (including the bright head lamps) on these older cars (70 Challenger)?
I was doing some head light testing today and noticed that with all of the head lamps are on with the car in D (825rpms), I noticed that my coil voltage dropped down to the mid 12's or so. This is still fine with my ignition system, but once it is in N, the coil voltage went back up to the mid 13 volt range around 975rpms with all of the head lamps on.
Is it typical to drop a volt in D with all four of the head lights on?

Thanks

Yes these old systems do pull a lot of amps.

The lights pull around 20 amps. Muscle era MOPAR charging systems generally couldn't keep up at idle with lights on so voltage drops until RPM increases a little. Its normal.

8 to 12 amps max if halogen... high beams maybe a little bit more an depending if is 4 or 2 beams. 4 beams tipically could get 20 amps consumption if Halogen. We must remember when you turn on the light are also turning on parking and cluster lights and you will be amazed how much amperes sucks the cluster ( dimmer them out and will notice! ) maybe an E body a little bit less since uses less bulbs than some others

Now, the voltage drop, depending of the alt capacity. But yes, you can get close to a volt. Thats why ppl changes to voltimeter because volts is a reflection of the amperes consumption, although I still preffer ammeter.

I replaced the alternator Saturday morning with a Powermaster #7508(75A) unit and it appears that it is doing its job. With all of the lights on including the brights, and the tranny in D with the brakes applied, I am still seing 12.5+ volts at the coil. I am running 4 halogen headlights and they are BRIGHT! I just wanted to make sure that this voltage range is correct for these old cars.
Thank you all for your replies.

The old headlights were 55/65 watts. The low beams pulled 55 watts. The high beams pulled 65.
Two low beams on pull 110 watts combined. 110 watts divided by 12 volts gives you 9 amps.
Toss in the high beam at 130 watts combined and you're sucking down about 11 amps.
All four headlights on tip the scales at 20 amps.

Running the headlights on relays will cut down on the voltage loss through the system as well as the amperage draw on the bulkhead and switch. The headlights will be much brighter too.
Here's a great write up on a hack-free way of installing headlight relays. You can put it together in your living room then carry it out and plug it in on your car. This installation is for a truck but the same thing works on cars. It's an easy step-by-step with lots of pictures.
http://www.dieseltruckresource.com/dev/installing-headlamp-relays-t176263.html?t=176263