You can gut the thermostat housing, AND plug the bypass hose running around the back of the motor. THat way all the water from the pump goes into the port water jacket. If you don't plug the bypass then much of the water will simply bypass the powerhead and flow out the exh elbow.
HOwever, the right thing to do is pull the port water jacket cover plate. MOst of them have not been descaled inside, so you'll likely see a few inches of scale from the iron block rusting over the last 40+ years.
I use a wet dry vac to suck the scale out of the block, and screwdrivers etc to scrape the loose flakes, then reinstall.
It makes a Huge difference in water flow.
You will see with the jacket cover off that the lowest cylinder actually has a passage under it that goes from left to right side of block. That passage is likely filled with scale.
This is all assuming you have good water flow to the impeller in a barrel/lake/flusher attachement.
Hope this helps.
ALso Dave, does your shift cover plate have relief holes drilled? IF not, you can pull the plate and see if that helps flow as a first step. I see this running in a barrel all the time as the exhaust pressure pushes the water below prime level of pump
EDIT: Dave, call me to discuss ok?