Adding onto an old thread here in the hopes that what I did may be of help for others who come across this.
When I got my car earlier this year ('69 GTO w/ AC) there was almost no air coming out of the vents at all even on the high fan setting.
1- One thing I did was seal up the lower AC ducting section that bolts up underneath the dashboard by using plastic weld cement in the insides of the unit. this sealed up all the seams and prevents leaks. I also sealed up all the ducting underneath the dashboard using HVAC tape. including where it meets the firewall. I found this tape to be very thin/ easy to work with and wrap around the ducting connection points to seal up everything. It's all great until I need to take things apart underneath the dash. Pro tip- do this after you think you're finished taking things apart underneath the dash
2- After that and still not being satisfied with the volume of air being moved I started looking into seeing if others had replaced the blower motor with something else more powerful and came across this thread. In the interest of wanting to try all possible options I too ordered the replacement motor recommended here (VDO/Continental PM136) bought it from Amazon, and when received I removed the original blower motor from my car. I then clamped each motor to my vice (one at a time) and tested them with an extra car battery and jumper cables. This is where the big surprise came in- the original motor was noticeably more powerful than the new one. the start up was, well, almost violent for lack of a better term and blew air all over the place. the new motor was like a calm morning slowly gaining speed and moved much less air by comparison. So back that went and the original motor went back in the car.
Now when I removed the blower motor from my car I noticed the cooling tube was missing- meaning that air was just blowing out the hole in the heater core box that should have been feeding air back to my blower motor for cooling. Also, while a previous owner added a ground wire they didn't bother to clean the paint off for bare metal contact. I fixed both of these things when reinstalling. I also then added a bead of RTV silicone around the fan mounting flange to ensure no leaking air when reassembled and then tested the fan motor once reassembled to feel for leaking air. I found a bunch of leaking air up inside the inner fender area where the heater box mounted to the firewall. I then added more RTV silicone to that to seal it up, then put the rest of the car back together and tested it the next day.
Significant improvement! Now all the air that the fan is moving is coming out the vents and not bleeding away into the inner fender.
3- Lastly, this weekend I've further upgraded the car with a higher amp alternator (10si 95 amp internally regulated alternator) and this too seems to help with the fan having more voltage to work with.
The big takeaways here are to seal up the ducting both in front of the firewall and underneath the dash to make sure any/ all air isn't bleeding away from leaks.
hope this helps