Question is why don't you have what you call "not so much airflow" in the existing system?
Airflow thru ducts or airflow by fan to radiator and condenser? That you can just upgrade fan, fan clutch or just a new one vs older they lose some % of power just by the years and miles - not that much for some. Some if fan clutch (not sure what was done for 1966 on trucks clutch matters a lot!
Couple types: Cheap isn't thermos reactive just a part point of strong all the time and annoying IMO with higher RPM use noisy and noticeable power loss to just fan.
Other is ask for thermostatic maximum duty that you can fit on it. May need to do some fussing for parts depending on engine. If V8 many full size luxo vehicles might just fit - Cadillac by '66 Climate Control was standard equipment for all (I think) and if not wanted a "credit delete option" was available.
If you mean airflow thru dash ducts - take out blower and clean the junk out or toss the wheel for new or whole motor and wheel together. Caution if you haven't been there or played with those, many of the fans are left hand threaded! Don't bust it or the motor is junk without it.
Hey - I see national news on how hot it's been about whole SouthWest of US. However, if it was adequate ever before for you, you might want to first fix the airflow issue.
IDK - it's at an age (plenty old enough myself to know of these new) nobody where I still live would think of paying for A/C in a real truck at all and never saw one with A/C ever!
It can convert and do OK. The two things that matter most will be fan and most efficient fan set up you can do then too,