Update: Fix is working good so far! Drove around for about 20 minutes and it was definitely nice to feel heat in the 240 for the first time in 9 years.. The JB Weld combined with the inner plastic barb connector is very strong. As a note: the JB Weld was more to create a seal around the splice, not to hold the pieces together. The plastic barb is holding the 2 pieces together very well. I did have to drill out the inside diameter of the new nipple just a bit as it was a little too narrow, but the barb fit perfect into the original nipple.
Those aluminum heater core nipples look great, but not so sure it's worth that much when you can buy an entire heater core with new nipples for $30, but to each their own..
I'll update again if there are any issues, but I think this will hold up for a long time.
Below are some pics of how it looks now. I'll also include a photo of the plastic barb I got to splice the new and old nipples together(only came in a 5 pack at my closest Home Depot):
