351 overheating

Ok trying to help out my mom and her Boyfriend. They have a 27ft 1973 Sea Ray with a Ford 351 model 233 engine ( not the original engine replaced in late 1990's)

It was over heating at the end of last season and they just put it away for the winter to deal with this year.

Mech at yard changed out the water pump and tubes in the out drive
Changed cap (electronic ignition)
New wires
New Plugs MR43LTS

Still overheating 180-200 at low RPM

I get involved,
I put new ex manifolds and risers
New 143 degree stat
New circulating pump
Oil change and filter.

Take it out and these are the numbers
RPM TEMP
1000 140
2000 142
2500 145
2600 160
3000 165
3500 170-175

Now for the Questions (trying to get technical answers from these 2 can be a trip LOL So I dont know what the numbers used to be.)

Are these normal Temp ranges for this engine at these RPMS ??
What Oil would be recommended for this engine???

Would the spark plugs make a difference????
The Original Spec for the motor was a MR43T

Right now there is a MR43LTS by the mech.
Looking up the spark plug numbers the one in there now has a longer reach then the original.

Would the shorter reach one make the engine run any cooler???
Should the engine run any cooler???

Thanks to all in advance.

 

thermostat?

 

already changed, going to go through it all this weekend looking for a blockage

 

Thermostat temp rating

The thermostat for that engine calls for doesnt open till 160. I would assume an engine in the 180 range is running about right. 200 + I would consider hot

 

Reach

If your talking about thread reach, it can mean the difference between a sweet motor, or a pile driven set of pistons, and at least a major over haul.
you use the reach recommended by the manufacturer, if burns cold(black, sooty} go to a hotter plug, and vice versus(ideat is a light fluffy tan). this is build into the nose of the plug! Also having too long of a reach can cause a hot spot and pre ignition, and run hot that way.

 

Ok guys this is what I found. Some were along the line someone spliced in a plumbing check valve and some nipples and a tee in the hose from the stern drive pump to the circulating pump to add a fresh water flush to the engine. Removed the set up and the flapper in the check was broken and stuck. So at low rpm you had flow but when you got it going not enough water was being passed through. The set up was probably 20 years old. My friend has had the boat for that long and it was always there. NOT FACTORY INSTALLED. So at WOT 165 temp. But once again I forgot the camera, so here is a really bad line drawing of the set up.
Thanks for all the help guys, I do Appreciate it.

 

The temp sounds right to me
Can you put your hand on the top of the manifolds when running at WOT?
Did you use an automotive or marine water pump replacement as some car versions have impellors that will make a MerCruiser overheat.
Make sure the bore of the hose from the transom plate to the thermostat housing is constant and not reduced by those extra fittings.
If it is too far advanced the Fords will overheat, index the balancer to be sure it is telling you the right story.