[Steve Barcak]

Was not able to figure out how to pull the heater box with dash and a/c ducting in place. Trying to r and r that passenger duct while the dash is in the car would be a night mare IMO.
I ended up unbolting the dash in its entirety but, lifted and slid out the passenger side leaving the driver side in place. I used 2 1" ratchet straps to suspend the dash. This was a bit easier ( if the word 'easy' could be used) with the r t top removed and attached the strap hook to an attachment on my ceiling in my shop. with the strip holding up the dash, I was able to slide out the heater box and replace the core.
I ended up using a used core! some may think that is nuts but, I have been collecting Pontiacs over 45 years now and have many Firebird/TA parts cars for over 30 years. Heater cores do not age when dry and not in use. I pulled a clean one our of a 79 TA and used it.
I do not know how many alum heater cores I have purchased and then had to exchange under warrantee. I'l never use an alum heater core again if I do not have to.

So, after this was done and the dash almost removed, I decided to dig deeper in the project and pulled the dash out.
Some years ago, I had a dash restored for the car and had no intention to install it until I painted the T/A but, decided to install the new dash since I was this far into this mess.
With all my decades of working on GTOs and Catalinas, this is really the first Firebird that I have serviced a heater core on.
I can say, everything on the Firebird is more difficult to service over a GTO or Catalina. So, I am trying to keep liking the car though I would much rather work on one of my other Pontiacs.
Will be glad when it is over. Having a heater is a good thing. We got more snow the last few days here in eastern Az.
thanks everyone for the advice and replies. Have a nice weekend!