I'd call David Kee and ask him directly if what you want is feasible. I don't know if the innards swap from 64 to later style cases, but he would. If so, it might be a good way to go. You may still need other parts during the rebuild, though. Again, David would be able to give you an idea.
Weight savings is great where you can find it. I imagine I shaved quite a bit off the front end with aluminum heads and intake. An aluminum tranny housing would definitely help (toploaders are freaking heavy), but I don't know how much, nor do I know if there are any durability issues with them (flange ears breaking off, etc).
Aluminum driveshafts are common nowadays, and also a great way to reduce not only curb weight, but rotating mass.