I don't think an engineer would make you fit a disc brake rear end for compliance. Is the 5.7 an LS engine.? If so its not going to be an all original van so fitting an O.E diff wouldn't be important. I'm not a real 9" diff fan, they are heavy, not as strong standard as many people boast about especially the Ford boys, and can get a bit expensive depending on the spec, but there are parts everywhere and a heap of quality aftermarket components for them makes the 9" very popular. If your going to run a T700, 2.78-1 ratio would be a good choice if choosing a 10 bolt. From memory HZ-WB statesman's had 2.78 with rear disc setup if you still want rear disc brake.