UPDATE:
Over the weekend, I installed some coolant flush Tees, clamped off the supply and return lines to both heater cores, and clamped off the supply and return line to the rear heater core. That left me with direct flow through the front heater core. I rigged up some hoses using a garden hose and flushed the hell out of it. After that, I used a drill pump and ran some radiator flush and hot water into the core until it came out the other end and let it sit for about 20 min. After that, I kept going back in forth with garden hose on both sides of the core, until i stopped getting debris out of it. I did this for probably about an hour of back and forth. After filling the core back up with coolant. I ran the engine. Immediately a difference in heat. I used to only see about 100 degrees inside the vent, but after the flush it was reading about 125-135 degrees inside the front vents. Granted the ambient air temp was 60 degrees, so we will see how well it will heat when its actually cold out and how long this will last.
I also did not get to do the rear as I wanted to see if there was an improvement in the front before I took another hour flushing a core. So I will move onto the rear core core in the next week or two and see how that one turns out and probably flush the front one more time just because. Especially since I have all the buckets, hoses and drill pump set up done, might as well just use it. I also added a small jug of coolant conditioner and rust preventive to try to slow down any further build up on the cores. Ultimately, if I can get a year of warm heat until I have to do it again, Ill be happy. Not that big of a job, just time consuming.
Still got a little more work to do and more flushes. But overall, it was a success at least figuring it out that the cores were the problem. Thank you everyone for your input!!
