120psi is about 8bar, so a tank rated at 10-12 bar is not overkill at all.
PLEASE make sure you use properly-rated parts. It is also a very good idea to do a hydraulic pressure test first (fill your pressure vessel with water and pressurise it) - a bit of water running out is far better than shrapnel flying everywhere.
Don't underestimate the potential dangers, 120psi is enough to make a big bang!