The heater core had been bypassed years ago due to leaking. I purchased a replacement core and set out to fix it. The new core, seen on the right, wasn’t as thick as the old. I thought this might present a problem, but it worked out nicely as the replacement mounting bracket that I purchased was taller than the old one. The old bracket was brittle and broke during removal
New mounting bracket:
The most difficult part was removing one of the nuts that secures the heater unit to the firewall. There is a nut behind the fender with limited access. I was able to get a wrench on it and turn it about 1/8th of a turn at a time. It took quite a while, but I finally got it off. I decided to not put it back on (at this point) as there are four other nuts that work fine.
Fun fact: Guidance for removing the tough nut is to remove the fender OR drill a hole in the wheel well. Meh.
I took the opportunity to paint the heater cover while it was out.