Personal choice if your going to drive the car IMHO - allot of 66- 67 V8s out there that have been used for 50 years without that pulley without issue
True. AC will function either way...with or without the "slap idler" pulley. Something to note on a scientific fact...The higher the "High Side" pressure reading of the AC system pressure, the more noticeable the "slapping" of the belt might be. (the slap occurs at idle speeds for most part). A person running alternative refrigerants should expect a HIGHER high-side pressure reading AND then add any potential of a higher ambient air temperature (say over 100F degrees), you could expect even higher readings than the Ford engineers were dealing with in the 1960's. I've experienced such belt slap on similar long belt drives.
Living 21 years in the Palm Springs area and servicing AC systems including being certified for retrofitting early cars to R134-a refrigerant, the experiences taught me a lot...and the one take-away I can offer is
FOR DRIVEN REGULARLY CARS, these old compressors work best if retrofit with the Sanden rotary style compressors AND you'll notice GREATLY IMPROVED FUEL ECONOMY while operating on these alternative refrigerants.
OBVIOUSLY, this is not at all Concours, so this Modification falls way outside the focus of this forum. A high efficiency condenser is a good idea too, they are now producing one that looks +90% original. There are ways I know to improve efficiency of the old retrofit systems that many are not aware of...MAXIMIZING the air flow through the condenser is the easiest way, but there are other ways too
Personally, I plan to fire my system up on a rebuilt OE Ford compressor with a modern refrigerant oil that is compatible with the newer 134a freon. After it has been determined to be functioning fine on the 134a, I'll recover out the 134a and shoot it up with R-12. I want the system to LOOK antique and original to the year of car it is, so the upgrade NOT worth the change in appearance for my desires.