'66 Barracuda having some charging issues

Hey guys, I went on a 2 hr trip up north for a car show last weekend and had zero problems, even in the 95* heat she ran nice n' cool. Got back home fine too. About 15 min from getting home I noticed that my Amp meter was in discharge while driving. Nothing was wrong that I could tell so I pulled the headlights on to check the meter and it dipped even lower into discharge. Reving the motor didn't do anything. I'm thinking my alternator must have died. All week was busy so just this morning I ran out with the volt meter and battery is 12.4V with car off and the same with engine running. I read another post here about jumping the single green wire on the alternator to the alt stud but that didn't do anything accept spark and make the car almost cut off.

One WEIRD thing I did just remember was that when I first started her up this morning, it did show Charging on the amp meter, just like it always has. After it warmed up for a little bit it was back to discharge. My gut says its the alternator . . . I can't think of anything else that would "fix itself" while the engine is cold then break again when it warms up.

I'm still pretty new to the early A-bodies so any tips would really help! I'm getting ready to just drive her up to the car parts store and see if they can test the alt or just buy a new one and see what happens (but keep the receipt). In the mean while I'm charging up my spare battery to throw in the trunk ;)

By the way, it's a '66 barracuda 273 4bbl top end with 360 heads/bottom, all original wiring accept for the Mopar electronic orange-box ignition. I've had ZERO problems with the charging system so far.