I like the idea of the air ride , I can drill and tap for schrader valves , will it affect dampening ????
it will make the forks return faster, much like a heavier spring will.
I don't pay a whole lot of attention to the manuals on the amount of fork oil, since they are not always the same from side to side. that can be due to one side draining out more than the other etc. I start them all at 5 inches from the top of the tube (measured from the "short side", or side closest to the rider), without springs. Next I extend the forks and make sure that the damper rods (in your fork's case) are covered. In go the springs. Now you can ride it and add oil if necessary. Some of my old damper rod forks were as high as 3 inches from the top. you get a certain amount of bottoming resistance from that alone, since the compression space is smaller.
For you , I would start at 5, make sure the levels are equal. do the drilling and tapping. you can get some really nice compact line that air brake equipped trucks use as control line and set the forks up with an elbow and a t, which will allow you to fill from one side, and keep the pressures equal. clever routing can keep it all behind the number plate/light if you want.
Try and remember the surprisingly low pay we get for dispensing what most of us learned by lots of time investment, not to mention money. (Manuals are pretty easily obtained online fyi.)