Just got the car from body shop and NOW have an electrical issue. Battery is fully charged but my ammeter gauge is fully pegged to the right even after 10 minutes of driving. How do I test to where the problem is in the system? Car is a stock original 69 roadrunner.

If the battery is fully charged, then next most likely is either something wired wrong, after that maybe the Regulator is stuck open.
The diagrams here should help you see where to look.
http://www.heritech.org/cuda/Charge.html

The bottom wiring will be similar to your '69 and shows the Voltage regulator wiring. (It will be best to check the colors and wire sizes, but I think it is about the same.)

Thinking about it further, I'd do this:
1 Look over the wiring.
2 Check the battery voltage not running.
3 Check it running and check the voltage at the alternator (bat stud).

Depending on what was found above:
4 Probably I'd check the voltage on both sides of the Voltage Regulator.

If the VR is showing full voltage from in (ignition) to out (field positive): Check that the Regulator is grounded to the chassis, and chassis to battery. Make sure the points inside the regulator are clean and not sticking.

Yes you are "full fielded" & it is extremely damaging. Do NOT run it more than 5 seconds at a time to see if the changes you are going to make solved it & the first change is to go to your parts house & purchase a new electronic reg. Most if not all parts house regs are now electronic & if you turn it over & there are no wire wound resistors on the bottom side (like the 69 OE unit would have) then it is an electronic unit. Even tho I dont like shotgunning parts at a vehicle to solve an issue it is highly probable that it IS the reg & they are problematic even when right & have a somewhat high failure rate so it is a good plan to replace the OE electromechanical unit with an electronic version. also when testing voltage at the battery (at a fast idle) to solve this have all lights/stereos etc OFF as this high of voltage will fry them or at least shorten their service life. keep your old reg & clean the contacts if desired & the OE black cover will fit some parts house regs for an OE appearance & a guy on here (mrrandyj) used to or may still sell an electronic reg with an OE black cover for $29 to your door. EDIT You said all original so I would get a new reg to be up & running (without frying things) to enjoy your newly painted ride then later you can clean the contacts/adj its tension as needed on your OE reg or buy a NOS one from eBay or elsewhere.

Sounds like it is over charging.
What alternator and voltage regulator are you using?
Stock '69 system or the system that connects to both alternator field terminals?

If it is a stock '69 system (regulated on the _12 power line to the field), and wired correctly, the most likely problem is that the voltage regulator is not grounded or is defective.
On this system, the single wire from the alternator field wire goes to the voltage regulator, and no other wires should be connected. If you disconnect the wire from the voltage regulator, the alternator should stop charging. It it continues (power on the field wire) then something is wrong with the wiring allowing power to by-pass the regulator.

If it is the later system that uses both field terminals, then the voltage is regulated through the ground side. Disconnect both field connections at the alternator. with the ignition "on", find the one connector that has battery power and re-connect it, leaving the other (voltage regulated side) off. Start the car, and it it still overcharges, the most likely issue is the alternator brush holder for the field connection that goes to the regulator had shorted to the alternator case. If no charging, disconnect the triangle connector from the regulator, and reconnect the alternator connection. If it now overcharges, the wire between the alternator and voltage regulator is shorting to ground (maybe a pinched wire?) If it still is not charging (it should not be), check for the ignition power at the regulator plug. If the regulator field wire is connected you should see power at both terminals, with it disconnected there should be power on only one terminal. Most likely this should check out OK. reconnect VR connector.
If you got this far and it still overcharges, the voltage regulator is shorted internally, so replace the voltage regulator.

Could be a huge current draw (short?) somewhere in the system. Start disconnecting things one at a time.

Being it was fresh back from paint, I would make sure all grounds are clean.
Most components like your regulator are ground to the sheet metal with the mounting screws.
Anything such as engine, charging system, lights other accessories all need a good ground

Agree. Check grounds, especially voltage regulator to firewall. Add a supplemental ground if needed.

As test, pull both voltage regulator (smaller) terminals off the alternator. Give it a start. If it still wants to over charge, the alternator itself is shorted.