Your going to get several different opinions on this and you will be thoroughly confused by the time this thread hits 10 posts...
Here is my train of thought. Not saying I am right or wrong, just saying this is the way I have always done it and never had any issues.
I run a several turn gate valve directly off the pump, from there I go to a splitter to both inlets of the block. At the thermostat I have two outlets. One goes to a dump line overboard, more on that in a second. The other line goes to a screen strainer then to my control tee (I have a Bassett but they are all similar) then to the headers. The dump line size is where you can make or break the entire system. Not the actual hose size per se, but the outlet fitting.
The set up works because of the gate valve. once the above is plumbed and ready to rock, run the boat and have with you either some bushings for the dump line, or a smaller piece of hose to shove inside the dump line before the outlet and an extra hose clamp.
Run the boat with the gate valve opened a few turns watching the engine temperature gage. It is imperative that this gauge works.
After the engine warms up take it for a short run, you will know right away if the valve is too closed (she'll start to get warm) and you will need to open it up a bit (I like to mark the handle of the valve with White Out so I know where I am). The header valve should start to mist the headers at about 1700 RPM (for huge camshafts about 1800) if too soon, close the valve a bit, too late, open it a bit.
Once you get close, then you can monitor where the motor will run temperature wise.
Typically, you are really only concerned about the temperature at idle, mainly because this is when water flow is the lowest. Try to keep it at or slightly below 160 degree F. Running WFO they will typically run at like 110-120 degrees F If the temps just wont come up, yu may need to put a restrictor in the dump line. I have used a 1" piece of fuel hose inside the dump hose before the fitting and a hose clamp to keep it in place, you can always use a hard piece of plastic so you can drill it larger if needed, but honestly think you will find that it will end up between 3/8" to 7/16" outlet.
I have done many way more exotic set ups in the past, and they all work in a similar fashion. This way just uses very few parts and none of them move.
One word of caution, use a zip tie on the gate valve once you think you have it dialed in, this way a passer by can just dick with your junk so to speak.
Hope this helps.
GT
