Yes people read the forum, although less frequently than in the past. We need more fun questions like this one.
I also have a 1976 Contessa.
The water tank was under the the port side quarter berth, positioned forward so that part of the top of the tank projected 2-3 inches into the galley sink vanity. The aft end of the tank was blocked in the quarter berth by a fibreglass panel about 26 inches aft of the galley. On my boat frost had split the tank at its bottom outlet. I had a grand fight trying to extract the tank without cutting the fibreglass of the quarter berth. In the end I decide to scrap the tank -- I cut it into about a dozen pieces for removal. I now have a Plastimo flexible water tank in the bilge between the head and the hanging locker. My thinking was to lower the centre of gravity a bit and free space for storage in the quarter berth.
If your water tank is gone then you have lots of options. For example Ronco has a very rich catalogue of ready-made tanks. https://ronco-plastics.com/
If you want to replace the tank with something similar to the original then you will have to make a larger opening in the top of the quarter berth. Remember that you will have to close this hole in order to sit down again!
If running twin potable water tanks I would valve between them. You want to avoid having bad water on one side contaminating the other.
If trying the bilge as I did, the trick lies in the hose runs. My cabin liner would not allow me to pass a hose from the bilge directly to the galley area. Instead I ran the hoses forward under the v-berth and then back through the hanging locker to reach the galley. It makes for long hose runs but has minimal impact on the boat and keeps everything hidden from sight. I was only required to drill between the hanging locker and the V-berth.
Since I enthusiastically complicate my boat (not recommended!), a tee at the tank feeds a second outlet aft through the bilge into the engine compartment where the electric water pump lives. My thinking was to make it slightly quieter and group all the mechanical systems in one place. The water line then passes through the port quarter berth and back to the galley and a very small hot-water heater. Hot water on demand is a nice luxury.
p.s. If you do not plan on drinking the water in the tank consider plumbing the sink pump to a sea-cock and carrying potable water in 4-litre plastic jugs. This is both easy and inexpensive.