I actually wasn't too concerned with it failing (which wouldn't be that dramatic if it did). More about how much the pressure vessel would expand.somewhere around 4000 psi.
6061 aluminum shouldn't have any problem with that kind of stress (safety factor of about 10).
Designing a pressure tester?
I'm out in the boonies right now but maybe when I get some free time at the office I'll build one. There's enough pipe and fittings floating around for most of it. I'll probably have to buy a pressure transducer and some flanges though.