On the intake manifolds on my 454's I have a "Y" that sits on top of the thermostat. The 2 hose's go directly to the Stainless marine exhaust. My water temp is too cold and I need to put stat's in. The Q is... If I put the stat's in, won't that cause problem's with the rubber fernco that goes from exhaust inside the boat and outside? Or will the stat allow enough water through to keep it cool till the stat's open?

Excess water that does not go into the engine will still go into the exhaust to keep it cool.

might need more information .....do these motors have Circ pumps or Crossovers ? you need a bypass hose from the Circ Pump/Crossover to the thermostat housing ....m

They just have the crossovers on them