I’ll toss out a suggestion in case you run into this.
The box containing the three dampers (defrost, panel and floor vent) has two that are pressed into brackets that connect to the actuator motor. Your dampers are likely rotted at the edges and the foam on the top of the box is probably shot as well.
Getting those dampers pressed off of the linkage is difficult even with the box sitting on a bench. Mine broke at the plastic, requiring either careful gluing or just replacing the box. I just bought a new box - it’s GM part number 52467204 and available on amazon for around $70. It includes the dampers already installed. The third damper that routes air to the floor just drops in and is easily replaced. I’d replace that as well and put some new foam on the top of the box.
Separating the top of the box without pulling it from the vehicle may be a challenge since there are several screws that are against the firewall. It fits together like a clamshell. I haven’t tried to do it without pulling it, but pulling it requires a coolant drain down and opening the a/c lines.
It can snowball if you pull it - the evaporator line loves to rust and seize where it connects to the accumulator and there isn’t a lot of room to work to begin with. In my case, I was replacing the entire a/c system so it didn’t matter, but it’s something to keep in mind.