Jason, as you well know, you will either get an answer from someone who knows for sure, or an answer from someone who is guessing or an answer from someone who can figure it out on the fly. You strike me as someone who can figure it out on your own given enough information. I would start by flushing the block, I hooked a garden hose to the upper coolant hose connector and took the water pump off and flushed a ton of rust / crap / sandy looking stuff out of my 57' 235 in my 49. This solved the running hot problems for me. My impeller on my pump was somewhat rusted out, but still had the basic shape and reasonable clearances.
If by putting a restrictor in your bypass, you elevate pressure in the block, you will prevent steam pockets that allow areas of the block to get hotter and hold heat.
Hope this helps.
