+1 oil temp is the main deal,,,,,,,,,never a hard run bellow 180 oil temp and never over 260 oil temp.

If you want to get freeky on water temp the block should run hotter than the heads (work that out) stick with the oil temp and all will be fine 2c...........boats are boats and cars are cars don't mix them up.

Ok. I will now install oil tstats. But I'm still a little unsure of how/if to install a water tstat.

Should I go for drilled 140 stats and a bypass? Should I also have a pressure relief valve?

Or should I just install oil tstats and skip water tstats?

Put the 140 Water stats in with 1/8 " holes drilled. (you can go bigger, in small incraments to help relieve pressure. Prolly up to 3/8) Start with a 1/8 hole. Also use the bypass, so water stat AND bypass. Then run the boat and watch your water pressure. You may not need the pressure valves. Im not positive on this part, but 25LBS is about max for water pressure. Watch the gauge cause pressure will come up with more RPM's. .....Oil stats will come in after you get this water temp/pressure dialed in and you still have cold oil. THEN do the oil stats.

Gauges, they will tell you what you need to do.

I just installed a crossover on mine. I also drilled & taped the back of the intake manifold and ran a 3/8 hose to the front of the manifold. I have not run it yet. I have heard people say it helps with the cooling.

I have a 522 BBC that I run pretty hard and I use a Stainless marine setup with a Stewart 160 high flow thermostat and it works great. The Stewart marine thermostat has the right size holes already in it. My gauge stays at 150 when I idle even after an hour and stays just above at 5000rpm. I will get a temp rise to 185 when coming into port after running 5000rpm for 20 miles. I am using a stock Merc seawater pump that I've ported for better flow. They come with a lot of sharp edges from the factory that will tear up the impeller.

Ok, thanks for all info so far. This is my plan:

1. Install an oil thermostat. Since I have decided to use oil tstat, this should be installed before finding the right water temp setup.

2. Install bypass, a 140 tstat and a pressure relief valve set at 20psi.

3. Run and see what happens with oil and water temp. Hopefully it will be good

As long as you have a press. relief valve and adequate sized hoses, (make sure you use large enough hoses) you should have no problem getting engine temp with the 140 stat. I have the Merc relief valves and resprang them for approx 22# max. I think they used 1" ID dump hoses.

Yes, running water dumps from the back of the intakes water jackets like you are doing helps cool the rear cylinders. I will be doing this also, but running the dumps right out the back of the hull with separate dumps. Avoiding steam pockets and to help keep the #7 and #8 cylinders cool. (blower motor)

I did the same on my last boat, front to rear