That one's a real "zero pressure" system. The best you can do without swapping radiators will be to install an overflow tank and recycle whatever coolant gets pushed out of the top tank when the engine gets up to operating temperature. You can check the accuracy of your temp gauge by immersing the sensing bulb that is installed into the cylinder head in a pan of boiling water and see what it indicates when the coolant boils. It may (or may not) give you an accurate reading on the actual coolant temp. It would be possible to super glue a thin piece of rubber gasket material to the current radiator cap to be sure there's no leaks at the filler neck.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!