You'll probably need one of those left-handed crescent wrenches to get the cap off. They can be kinda tricky.
The tank is mounted on the housing that covers the wobbler wheel. Be careful when filling because if the wobbler wheel gets wet, it will throw it out of balance and not wobble any more.
When you find it you will also be able to find the piston return spring and the radiator cap for a 1966 Chevy Corvair.
If you go to an aviation supply warehouse, you can get it for half the price as at a truck stop. It's right next to the prop wash in isle 3.
A Special Forces Ranger Seal told me the Federal Marshall's made it mandatory truckstops give it to you free if you fill out an id10T form
Check form ID-10-T. Tells you on that form exactly where to fill that fluid. It's in your permit binder.