I have installed a metal plate with 1/4" dia hole to restrict water flow in the water pump bypass tube.  This is on 292 Merc with recent rebuild.  Though it never boiled over, it always seemed to radiate lots of heat and the not so reliable original 6V gauge showed hot.  Checked with thermo gun and it registered 195 on top of engine.  After installing the restrictor plate thermo gus now shows 175 degrees on top of engine.  What is the down side to this arrangement?  Car has a 160 degree thermosthat.  Thanks for your input and many thanks to the person that suggested this on y-blocks forever!!    

Gary