Hard to say for sure without knowing the horse. Some people do drive their taller B's in the 48" shafts. Personally I do not--it doesn't have anything to do with refinement of the horse, though that could be an issue sometimes--a very big built 37-38" horse or one that is very overweight probably won't fit too well in the 48" shafts because they won't be wide enough.
What concerns me mostly is the length of the shafts--and I know that I don't want to be driving my 37" horses, even my 36" horses, in the 48" shafts. They are just too short for big moving horses--if my B's were to hit a full road trot they are almost certainly going to hit their heels on the basket of the cart. I don't want to have to be thinking of that & watching our speed (all right, I admit that having grown up riding and driving Morgans, I love a great big trot & Mom & I enjoy our trotting races--used to have a lot of under saddle trotting races when we'd ride the Morgans, and now it is trotting races in harness with the Minis!!) and so we got 54" shafts for our EE cart.