Thanks again for all the help, everyone!
Here are pics of my fix and the solution for any future reference:
First, a close up picture of the inlet side of the heater core. There is a valve there and you'll have hot coolant flowing through the heater core all the time, which will add some heat to your cabin when you don't necessarily want it. Thus, I was prompted to find a way to keep that inlet nipple, if I could.
Below, I used some painter's tape and oscillating tool to cut off the damaged part of the old nipple.
Lastly, cut off the end of the new nipple and spliced to the old inlet nipple using a 1/2" plastic barb fitting from Home Depot.
I added J.B. Weld around the splice to prevent any leaks from occurring. The nipple is now a bit longer than stock, but that's okay with me
I haven't hooked my heater core back up yet, but I got the dash put back together and will report back once I get it all hooked up to let everyone know if it worked or not. I'm pretty confident this will work though..