i think if the mounts work, then you would end up with an extremely vibey bike. the a engine has rubber frame mounts to isolate the rider from the vibes, and the b engine has an internally balanced engine. what you want to do would effectively do away with both of those...
Get ready for a paint shaker. Ride one of the old Evo SofTails (solid mounted, unbalanced) and you will quickly change your mind. Strictly a short range bike.
Shouldn't be any real trouble. The rubber mounts didn't come out until the mid to late 80s. Everything prior to that was solid mounted. And the vibration wasn't that bad in them. In fact, that rumble you felt when you cranked the throttle open let you know you were on an American machine. And it sure felt good.