First of all, welcome to FABO there, Sam!! Now, I see no reason not to be able to fit A/C, power disc brakes and anything else under the hood with a 360! Dimensionally, the 360 is exactly the same as a 273, which was an option in 65, and could be ordered with A/C and power brakes! Physically, its totally possible to run all of the year correct components on a 360, but as you are probably well aware of, many of the newer components such as A/C are much smaller than the OE stuff, making for easier installation, increased efficiency and ease of maintenance! Sure, it costs more to buy the latest A/C systems, but when you weigh the cost against finding original 40 year old parts hat no longer work with the current fluids available, and are vastly inefficient in comparison, you'll find that its really better to by the new system than try to use the old!!
Now, duals are certainly a must for a 360, as you will be coming up against limited exhaust options! The stock V8 mainfolds do not port match with the 360 heads...headers are expensive for even the shortys, and unless you go the Dakota OE manifolds from mid-90's pickups, you are going to be giving up a ton of $$$ or power running a Y-pipe, which will not be correctly bent for the Dakota manifolds anyway!!
I make a trans crossmember conversion that allows duals to be run much the same as the later A bodies, and ill include a pic for reference! It allows whatever exhaust you run to pass through the crossmember area without having to go under the passenger side, which is not even cut out for clearance! Whether you decide on headers, Dakota manifolds, or something new nobody has heard of yet, the crossmember situation would still need to be addressed!!
Or....you could keep the little 6 and hop the heck out of that thing!! The Cox Brothers racing gang is running high 9's with their slanty!! Anyway you decide to go, you have a great little car, and one of my favorites!! Am I biased?? Maybe, but I love early A's!!
Have fun with your new car and welcome again, Geof