Our 63 GT also had a large cut out of wood - and yes the black tank is supporting the toilet - but the tank is made of fiberglass. the area is still supported by cross members as well as some floor - not to mention that the tank is integral to the gelcoat top finish of the tank. This unit is then riveted to the back wall onto another support piece. So in essence there is plenty of support. The wood cut out matches the length of the "giant" brass pooper fittings they used back then - so if they needed any servicing you would not have to cut the floor after the fact.
I wish I had a pic of the hole in our floor for you - perhaps this one from my thread called flight of the ladybug http://www.airforums.com/forums/atta...8&d=1180277232
The square piece of wood sitting on top of the floor is about the size of the hole in the floor.....
If you are replacing with a poly tank then you will need a bit more support - as the poly tanks are not as strong as the fiberglass ones.
Good luck Dan and Lisa - if there is any other pics or info we might be able to help with by all means PM us -we have had this baby apart 3 times so we now where pretty much all the wires and plumbing are running too.