My cuz just put an OEM (looks like)reproduction battery in his 71 340 Dart. Now the car is not charging .I didn't want to dig into it without knowing a few things . First thing I thought was the battery was polarized wrong and probably took out the regulator. But checked with meter and it was charged correctly. there is voltage to the terminal on alt.
Maybe there is a brush stuck or something simple as the car is totally restored and is not a daily driver. Any quick checks before he pulls the reg or the alt?

flat later style reg/2 field terminal system I'm assuming. eng off/key on see if you have fire at the blue field wire/terminal at the alt and at the blue wire at the reg triangle connector. Turn key off then see if the green field wire has continuity between the end at the alt field and the end at the reg triangle connector. All light bulbs/accessories off, Start it up/fast idle & touch a screwdriver to the back center of the alt & a magnetic pull will you it is charging (at least some). Pull the green field wire off of it's alt field terminal and jump the alt terminal not the wire you removed from the alt terminal to ground for several seconds max & see what you get for voltage at the alt "batt" terminal to ground. This'll tell you if & how good the alt is charging. Holler back. EDIT which everything hooked back up to normal take a jumper wire with alligator clips & touch one end to ground and the other to the reg case to confirm it is grounded (fast idle). You'll see sparks if it ain't or the eng will slow down or you'll see it on the meter

Done deal Robert, after I got the key. Not much I could do without the keys , thought there might have been some tricks I had forgotten . Anyway did as you said and new VR was gone and now replaced. VOM is showing 13.95 at fast idle and battery is not fully charged .
Thank you for the info .