ECE (Early Classic Enterprises) offers a 4/6 drop kit. Check out the
drop pics on ECE's website and that will help guide you for the look you are after.
ECE builds some great products and know these trucks inside and out. I plan on purchasing the 4/6 kit for my '72, but swapping out the 1.5" drop springs for 2" springs for a 4.5" drop in the front.
As mentioned above, with only wanting a mild drop, I wouldn't go with a complete air suspension. Maybe just a rear air bag setup, then if you decide to build on the air system later all you have to change out is the front, and possibly the compressor, tank, and valves.