With the weight oil your using and a high volume oil pump that was probably used in the rebuild. 80psi is not out of the ordinary. I would run a manual test gauge right at the engine just to verify what your dash gauge is telling you and how accurate it really is at lower rpms.
I do believe I'd repair it the correct way as opposed to the above statement and frying batteries and other potential issues with 16+ volts.
As Dave said it;s the voltage reg. "and or" an internally shorted battery with that type system will do the same.
That engine was equipped with a warning horn in the control box. The temp sender to activate it is located on the upper right corner of the cyl. head cover plate. The older systems do Not have a beep when you first turn the key on. The only time you'll here it is if the engine actually does overheat. (providing the sensor & wiring are still ok)
Mercury Quick Silver Mirage Stainless prop.
No dings, No marks, Like new cond.
Part No. 48-13702A4
14 3/4" x 21P
Fits: Mercruiser Sterndrives
R, MR, Alpha, Alpha Gen II, Bravo I
$200.00 pick-up or plus shipping
Retail is over $350.00