Michael,
Morse at Vintage brake outlines some of the reasons. I'd put it this way...I think a 160/350 hibred would be fine for a 160. I think a 160 brake with race compound would be ok for a FCB 350. A 160/350 hibred with race compound would probably be better than a 160 brake...on a 160.
The pivots, tolerances, wrap around brake shoes, and general rigidity seems considerably better on the 160 than the 350 (to me anyway). If you really want plenty of brake, use a 305/450, they'll stand a 160 on the front wheel. Even with Vesrah shoes. Also the leverage ratios on the 160 seem better (longer brake arm I think).
JohnnyB