Agreed, CX500 tank.
I've got one of these that I'm fitting to my CB750 project. These were originally mounted at an angle on the CX500, so the petcock angles rearward when the tank is mounted flat. I'm cutting out the stock petcock bung and welding in a Pingel bung...not sure if you could use the stock CX500 petcock at that angle or not.
It clears the frame rails for the most part, but the tank is much wider at the front where the rubber mounts fit; modification there will be necessary too.
Also, the filler ends up on the front downward slope when the tank is mounted flat, I'm using a different filler with the hole moved rearward. It doesn't look as though you'd be able to get the tank more than 3/4 full with the stock filler location.
Here's a photo of how it looks just sitting there: