Steve, I have the same design as yours but no heater core, am taking heat from back side of oil cooler aprox. 1/3% of cooler hot air to cabin, do not know if this will be enough, am in MI so will need good heat.   With bypass hole plugged the non-bypass therm will need holes drilled in it ( Lynn Hanover ) for coolant flow at cold temps when therm not open, correct. Just thinking out loud.  David  R.Cook    RV6A  Rotary.