Lotta people on the Sport Trac site run 285's on the stock 16x7 wheels. You will need to turn up the torsion bars in front 1.5 inches or so. Some people run spacers in the rear on Sport Tracs, I don't know if a Ranger would need them or not. I have 265's and the rears rub a little on the inside.
My .02, yes they will work on the stock rims, yes they would be better on 16x8's.