I personnaly don't like air shocks. I have 4 1/2 leafs on my 65. I have read and been told that the the 5 leafs are a pretty stiff ride. You can have people in the back seat with out to many sagging problems with the 5 leaf. Do you race the car alot?
Ditch the air shocks.. Unless you tow (load Leveling) or need them for tire clearence. Air is a poor damper, It will hurt you in traction and ride. Get a good set of Low pressure or high pressure N2 shocks. KYB comes to mind.
Can't help you on the 4 or 5 leaf
I put Heavy Duty 5 leafs on the 66 coupe because I had 3 teenagers. I wanted to have them in the rear seat and not have the rear drag. Well it didn't! Even with the kids there was no give. Without the kids if I hit a bump the speaker grills pop off. 4 leafs is plenty! I would not do the 5 leaf or 4 and a half given what I know now.
I have the Canadian Mustang 5 leaf rear springs and KYB shocks on the Fastback. The front has Branda 620 coils (a thicker spring seat pad to regain some height lost to the 620 coils), KYBs, and a 1" swaybar. The rear tires are 245/60 15s and the fronts are 205/60 15s. As others have stated, the ride is stiff, but I don't mind it. At highway speeds (and higher) the car is incredibly stable and smooth. Personally, I wouldn't trade the control and handling benefits for a better ride at low speeds; but I don't use the car for everyday transportation either. The car has a slight "rake" due to the springs and tire combination and has ample clearance for the 245s in the stock rear wheel wells. I could scan a picture and e-mail it if you would like.