Carl,
I recently primed my motor with a gauge on it, the port above the fiter pad on back of the block. With a Milwaukee drill set to 1200rpm, I saw 82psi at the back of the block. That was with the remote filter adapter with a bypass hose on it. So yes, you can get enough psi to test. It would be good to know how much psi each motor makes with the same drill, and setup.
If you are bypassing oil into the pan it will get hotter, cause not all of it is going thru the cooler.
When I had my thermostat problem, I would see 270-280* after the cooler. If you see cool temps after the cooler, how much heat is it taking out? If it is cooling properly the oil that is going thru it, but the pan still gets too hot, I would suspect a restriction and oil going thru the bypass on the pump. That restriction would be after what ever point you are checking the psi..
Are you running the same lifters in each motor.. They could be restricting the oil flow if they are not correct.
Keep looking..
I pulled a pan on one motor and ran the oil pump with a drill using a coffee can of oil to the pump. Found my leak.. the professional motor builder put standard bearings on a .010" under crank and never measured it.
If you have to, pull the pan and rotate the motor with the starter, and supply oil to the pump, what ever way you can.. then watch psi and flow.. if it is coming out of the pump bypass you should see..

Hope that helps.. Keep at Carl, you will find it.
Dick

Yes, you can use a primer and a drill to test, just have to do the same for the "good" engine to compair to....

The reason I ask all those questions, I had a problem like yours or it seems the same. It drove me insane. I don't want to say this is the problem, but try this.
Cap the by-pass. Remove the thermostat. Make a restrictor plate from sheet metal with a 5/8 hole in it or gut the thermostat. Run the boat, you need to check the engine temps with a temp gun first and during the test with the changes. The water temp is not going up because, the water around the sender is cool, what happends is the water at cylinders 5&7 get hot because of the flow around those areas, plus 5&7 fire in sequence, steam pocket produced, which causes the oil on the head to get super heated. You'll know the valve cover will be hot as H-ll there!
What happens if this is the case, the water coming from the by-pass sprays on the top of the thermostat and confuses it on opening, this all seems to effect the odd # bank. I moved the by-pass hose to the center of the intake and sent that warm water to the top of the thermostat housing. Take the front of the intake below the thermostat drill and tap it to 1/2 npt, use a #10 AN and your finished. Just cap or plug the cross-over or read your water pressure at that point.
From now on I use nothing but, the cast thermostat housings from Stainless Marine, they have a built in diverter to prevent this issue. I have used Stainless in the past with no problems, I used Eddie Marines, this was the problem. I called them about it once I knew the problem, their anwser was to drill holes until the issue went away. 800 HP heat doesn't go away with drilling holes in a thermostat! If this is the problem this is the answer to correct it. Or buy a housing from Stainless Marine, this goes for both engines if so the case. Let me know your test results. Mark