You can't connect a gauge to a thermostat-only to a temperature sender.
An 88 is a clamshell cowl mariner isn't it or is it a derivative of the yam-based mariner? I've got a manual here for the clamshell cowl version (in the garage-I'll check tomorrow to see if you've got a thermostat).
If it IS the clamshell 60 then there should be a take-off on the cylinder head water jacket cover for a water pressure gauge.It looks like a brass plug around 11mm in diameter.
If you can fit a temperature sender in there it'll work. If not you could get a machine shop to drill/tap you the right size hole for a sender.
A question-when you run it on flush muffs and the telltale stops and starts does the water coming out of the centre of the prop do the same? If not it could simply be a blocked telltale.
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