How large is the by pass hole under the thermostat in the water pump housing? Most are too large and will allow too much hot block water into the cooled water stream and cause some weird stuff to happen. Especially at low engine speed. Drill and tap that hole for a brass pipe plug. Then drill a small hole in the plug. Test drives will determine the end size needed. You can remove the thermostat and reach in to drill the plug without too much trouble.

But the hole can't be too small either. It allows the thermostat to 'see' some hot block water and keep it's operation consistent.

Motor home thermostats originally had a plunger on the bottom of their thermostats that would cover that hole when the thermostat opened up.