If you need R-21 I can probably get it for you. I just did my 73 and took it all apart and switched it to a new refrigerant called Dura Cool and it works great. btw test the heater core before you install I have found two leaks new out of the box. Also you will need a new foam kit for inside the heater box and also the seal that goes around the lines going through the firewall. The O-rings you can get at any auto supply to switch the refrigerant. You need to drain and change the lube in the compressor also. Clean all the o-ring surfaces with fine steel wool and change the dryer which I am told is also the filter for the system.
I have attached a couple pic of what I also do to keep the critters out of the heater box. You will probably find acorns and crap and maybe a nest inside your heater box. Along with all the clips is has sealer around the joint. You need to pull out, wash and clean thoroughly and do the foam and add the screen wire to keep the critters out. You can also reach up through the hole in the cowl and clean out stuff in there and add some stainless steel scrubber pad to the drains on each side of the cowl to keep the critters out. Also put the wire mesh under your cowl vent plastic pieces like in the pic. No uncommon to find nests inside the cowl or heater box or bodies, lol.
Pulling the dash partially out is easiest way and take the passenger seat out. Not that big of a deal just do it.
You can purchase the vacuum pump and charging gauges at Harbor Freight for less than what they charge at a garage for doing the evacuate and recharge.
It is just nuts and bolts do not be afraid of it. Go to the West Coast Cougar sight and watch their video on how to rebuild to prevent breaking anything. Also be very careful with the connection to the drain hose going through the floor easy to break heater box there for sure.
Take you time and it is not a difficult job just not the easiest for sure.
If you take the whole dash out you can remove and wash all the ducts and clean everything 40+ years of mold, dust and crap in there.
You can check all your vacuum motors with a brake bleeder. They need to hold a vacuum or they are junk and need replaced. You can also check your lines with the bleeder also. Might have rust hole in the tin can under the hood also so check it.
David