Hi Bug,
I was in this position a while ago - where a call for papers specified no word limit. A colleague advised me that proposals should be kept to less than 500 words - so I submitted about 400, and my paper was accepted. It was a very high profile conference and they can't have been too desperate, so I think this was good advice!
They will be reading many, many proposals so I reckon short and to the point is the best approach, and 350 words fits that bill.
Hope these words help. 8-)