I play in a highschool gym (where my church meets) every weekend, and a 320 watt head (Gallien-Krueger 700RB-II) alone is able to stand up against a drummer, electric guitarist (with distortion), acoustic guitarist, pianist, and a few vocalists. So, with PA support, I'm pretty sure 350 watts will definitely hold up with the rest of the band.
As for the echo/reverb/muddiness that you will probably get when playing in a gym (unless yours is acoustically treated), lower the bass frequencies under 100 hertz. I had a terrible time working in gyms due to the bad acoustics, but after Norm Stockton gave me this tip, it made a world of difference. With these settings in a room where the bass frequencies are bounced back and forth (causing muddiness), you can turn up louder without your bass sounding just like low, annoying noise. Thus, you sound a lot better than when playing in a gym using "normal" settings.