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The temp gauge is mechanical, and like mentioned is located at the rear or the head, just below the valve cover, on the right side.
I suggest one designed for the tractor, as opposed to a generic one as the "wire" is the right length. The generic ones leave you with a bunch of extra cable to tie up.
YT has one like this...http://www.yesterdaystractors.com/Ford-800_Temperature-Gauge-Black-Rim_C3NN18287ABLAC.html