You don't say which year (early with the 2 small grommets hanging on pins, or the later with the large rubber doughnut encapsulating the center bearing). If the later, I'd think that'd be sufficient. You're keeping the smaller (than what would have been originally mated to the larger diameter drive shaft on an M46). If you wanted to go to the trouble of getting a later, larger diameter drive shaft and modify it so that you can use the larger flanges on the front, but maintain the smaller flange on the back to mate up with the differential, the support for that might be heavier - I don't know. It might be that a drive shaft out of a later 1800 might have a support that is a bolt-in to a 122 tunnel. Others may know.