Sorry. 4 1/2" long arm. I have ground as much as I dare off the manifold but each time I get back from Moab there are new scratches. I never hear it so I don't care about it anymore. If you think about it, you are going real slow when flexed out that much.
I have heard of guys running them of of paint ball gun tanks. Not sure but a simple regulator should work and get you though a week easy. And you could carry a spare.