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OK, so the base gaskets are different? This same type of difference occurs on the smaller engines when comparing thermostat vs non thermostat models. It doesn’t make much sense to me. I would have thought that the non thermostat models would have a bit of a restriction “acting as a crude thermostat”, and that the thermostat models would be unrestricted. But, it seems to be just the opposite! So, by all means enlarge that inlet hole in the gasket.
One other thing to check for on these powerheads. There is a little coolant bleed hole underneath the powerhead that bleeds a little coolant from the water jacket to help keep the exhaust housing cool. These holes often get plugged up with carbon. So, please find that hole and make sure it is clear.
We all appreciate you taking the time and “risk” to experiment with this conversion. Just wish there was some way to mount those cylinder head cover mounts from the old head onto the thermostat head! Let us know if you need anything! Don