When I had my stock tank, I had a quick fill valve from HDG Products. I worked flawlessly, and would recommend it for you. No more than 10 seconds to fill up!. I actually still run it, and have the option of running that valve or with an dry break adapter for a dry break tank. I don't think making the tank into a dry break is a good idea, but then again, if anybody can do it, you can. Yes, if you put a dry break, it has to be vented.
The backing ring goes into the tank, then the cork gasket goes on the outside between the tank and receiver, tighten the bolts down to hold the receiver in place. I have a spare gasket laying around so I took it out to my stock tank, looks like it would work to me. Just need to use the gasket as a template to cut out the middle and drill the screw holes. Doesn't look to hard, as long as the mounting area on the tank is perfectly flat, which it looks like it is. I may just try it myself one of these days.
Although I bet the sytem Ceballos posted would probably work just as well, especially if you already have the 5 gallon jugs.
I contacted Clarke awhile back and they said I needed 41/2" flat surface. The stock tank aint far off that .
Can you buy the tank vent as a kit ?
I am guessing this is not something, I want to build myself.
I am sure they make a 90 degree one also, just found the straight one first. I then go to the local auto parts store and get a length of rubber vacuum hose, attach it to the vent fitting, then run it down to the bottom of the frame.