ABSOLUTELY change the poppet and thermostats!!!!
You need to change out ALL water lines/hoses as well, including the telltale hose (pisser)
If you're serious, temp gauges should be on each cylinder bank, with the sending units at the top. Keep the pressure gauge. Older pre-99 Merc V6's require 8 lbs of pressure at idle to prevent overheating and steamlock, and 18-40 lbs at WOT.
Muddy, any Mercury control box from from 1968 through 2002 will work. The Mercury cable system was unchanged throughout those years. FYI, All analog control boxes from 2003-2008 will work as well. You just can't use the digital SmartCraft controls.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury