Well just extrapolating from the dimensions you've provided, I'd guess you have more than 3000 cubic feet of air to move. Again, I've never heard the 350, so I don't know for a certainty, but I'd guess the 350 would be alright, but the 500 would be better, if it fits your budget. If it doesn't, go with the 350 and don't worry about it.

Please keep us posted on how things go.

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton