I've seen too many "sidewinders" while I've travelled in my previous vans -- they make my vision lopsided traveling behind them and I think I too am sidewinding ! -- can't wait to pass them and get back to normal !
The ideas posted suggesting finding points that are present on both sides of the body and moving the body where those measurements are pretty much equal. Naturally I'd first install the bushings and mounting bolts first, snug them down when you're satisfied the body is resting where you believe and see it's centered.
Google ford body builder for you year van .. it will show detailed info on frame measurements.. if you cant find it I can send you a link when I get home .. good luck