Fords generally have a big, heavy, one-piece drive shaft designed to slide in and out of the trans rear seal as the rear axle's movements require -- you will have to take the front half of the M-B shaft and the center support bearing and adapt the shortened Ford shaft to the back part of the M-B shaft, because the weight of the Ford shaft would probably be too much for the M-B transmission casting to support, even if you could adapt the Ford sealing system. Furthermore, those M-B driveshaft parts might just be the weakest link in so far as dealing with such a heavy gross weight is concerned.