If that's the fitting your going to use then the centre distance is fixed and you will have to use the bottle size it was designed for.
I made mine in two parts. The first part just looks like the screw end of a standard cylinder. The second part is a block of ally that bolts to the first part with 3 M5 screws and an "O" ring trapped between to give an air tight seal.
Then there are 3 blind holes into the ally block one 2mm dia. from the centre of the "O" ring.
A second from the bottom of the block upwards which meets the first one, this one is tapped 1/4 BSP and takes the filler.
And the last hole goes in from the front and is tapped 3/8 BSP and takes the 400 cc cylinder used in such as an Air Wolf.
The main advantage with mine is I don't have a filler stuck upwards spoiling the lines of the gun.
On my first gun I did fit a gauge in the same place as on the air wolf but found that with getting over 200 shots per fill I never looked at it. All I had to do was fill my gun when I first arrived at the range and not bother for the rest of the day, so on my second gun I didn't bother, why do more work than you need to.
I'm going to load a picture of my second gun to my album so check it out.