I replaced the brushes in the alt. of my '70 Challenger 340 due to it not charging. Now it is charging but boiled the battery and made a mess. This was not a problem prior to the no-charge condition so I dont suspect the VR and want to be more thoughtful than to just start replacing parts. Thanks for your help!

Check the output with a voltage meter, it should not exceed 14.6 volts.

if it does, do I assume a bad VR?

Yeah... good question... I had a reading of 15.30 at the alternator in my Duster, no matter which of the 10 voltage regulators I had was installed....I asked about it here but got no help.

I'd have to brush up on my electrical systems cause I mainly just build the components and rarely install them but I don't believe the vr has anything to do with the alt output.
Check voltage at alt, then check at batt. to see what the vr is letting thru.

The voltage regulator grounds the field to control the output of the alternator. You can test is by disconnecting the field wire and grounding it while slowly slowly increasing the RPM of the engine. Be careful to not rev too high you can destroy the alt and battery. Although it sounds like that has happened already. If there is something shorting the field wires such as rust or something conductive you will get an overcharge condition like this. It sounds like the insulator may be broken on the field side shorting it to ground maybe. Good luck

Overvoltage at the battery is due to one of three things:

Bad regulator. Pass transistor that controls the field has a shorted collector. Replace the VR

Bad grounds. Engine, battery, body grounds must all be solid. The regulator cannot function properly if these grounds are not all good.

Shorted field in alternator. Usually occurs at the brushes due to improper installation or missing insulators. With both fields disconnected, there should be at least 10 megohms of resistance between the field and the case. The 10 megohms is leakage current. Leakage current is part of the measurement set up and not anything to worry about. Digital meters see this more than old analog meters and lead to some confusion in troubleshooting.
Hope this helps.
Craig

Since you disturbed the system by replacing the brushes, I would start by checking your ground field on the alt. Is it possible a wire may have separated?