Here's some practical tips on AC.
First, make sure you have everything insulated properly and I don't mean the factory insulation. Here's a link on how to do it the right way.
Make sure you have all the holes in the firewall sealed well in order to keep engine heat out of the cockpit. Pay particular attention to the evaporator box and how well it is sealed to the firewall.
Then make sure you do not have hot water circulating through the AC system. Many C3s have factory installed shut off valves. My 74 did not, so I got a brass shut off valve from home depot and installed nipples on either side and then installed it in the 5/8 inch line going to the heat/AC unit. It only takes a minute to open the hood and flip the lever to shut off the hot water.
(I discovered this next tip while working on my system. I developed a tech tip article about it and posted it on the other forum. It was also used in a issue of Corvette Fever magazine about a year ago. It really works and improves a C3 AC system more than all the other tips posted. )
One of the biggest problems in C3s is hot air entering into the intake of the system. The system is fed by a plenum that is behind the rear of the front right fender. Here's a pic of it (BTW, this is not a pic of my car. Its an earlier C3 as you can see by the wiper door mechanism but they are all similar).
As you can see from the above pic the area that comprises the plenum is sealed by the ridgid body adhesive that holds the fender to the body. This adhesive over time dries up, cracks and sometimes falls out allowing hot air to enter the plenum. Also note that the lower area has a rather large opening to allow for drainage that is right above the exhaust pipe. You can imagine how much heat is drawn into the system given this design. What were they thinking? :crazy: In order to seal this area you need to take the right side kick panel off and the recirculate flapper. This will allow you to reach inside the plenum area and seal it. I used windshield mounting tape to do this. Seal the entire area that is shown with the yellow line. At the bottom stick one or two straws up into the area and seal up to them. Then pull the straws out. This will leave you enough of a drain that any water will have an exit but not enough to appreciably pull heat into the area.
There have been many posts about mounting a C4 fan to increase flow. The problem with this is the C4 fan requires a spacer because of the wider fan. If you use a spacer you effectively allow the same amount of fan to pull air that a C3 allows although the C4 fan may turn faster (no experience here). In my case, after the original fan gave up the ghost, I switched to a replacement fan (Hitachi I think) which was very reasonably priced and worked much better than the original. You can get replacements at most parts stores and they are quite a bit better than the old units.
There are two splash shields mounted at the rear of the front fenders. These units should be in place because they effectively create a heat barrier between the exhaust manifold and exhaust pipe to the body.
Make sure your AC system is in good working order. I don't mean just check the freon charge. I mean check that your condensor is not clogged and that air passes through it easily. Open up the evaporator box and make sure there isn't a ton of leaves and debris in there. Also make sure all of the flappers and seals are working correctly. Dr. Rebuild sells a kit to refurbish the evaporator box containing everything you need.
Make sure all of the duct work under the dash is sealed. If they are not a lot of air won't make it to the vents. I used metallic duct tape to seall all the ducts in my car so they are air tight. Also check the seals at the vents to make sure the air is all coming out and not being blown back under the dash. Remember that these ducts are in many cases over 30 years old so they may have cracks in them. Seal everything as well as you can.
Now, after having done this, just realize that early C3 systems were no big deal as far as air flow. Later C3s (78 and later I believe) had redesigned ducting that helped a lot. If you've got an early model you're pretty much stuck with the limitations of the system.
Now, after owning my car for 20 years and having gone through all of the above, I'll make a suggestion. If you want to keep your car completely stock or if you just want to maximize the system you have, do all of the above. If you have a car where multiple pieces (compressor, condensor, evaporator, fan motor, etc.) are not functioning correctly I recommend going with an aftermarket unit for the following reasons. They are engineered to modern standards and include a compact design, high blower capacity, modern cross flow condensors, compact compressors, electronic motors to move the flaps, and work well with R134. They also will cost less to install than the price of the major components of the stock system listed above. Had I known the amount of work and money I would put into my car to get the system working acceptably I would have gone aftermarket in a minute. I still may do it but its not a priority since my system works sort of OK.
Sorry to be long winded but its a complex topic and real world experience counts.:thumbsup: