When a motor is cold & the thermostat blocks coolant flow to the radiator, there must be a way to recirculate coolant from the block back to the water pump suction, if not then the water pump would be pumping against a "dead-head", the engine will not warm up uniformly, and steam pockets would form in the coolant passages of the cylinder heads.
Most engines have a small hose that routes a small amount of coolant from below the thermostat into the water pump. This recirculated coolant is flowing through this hose all of the time even when the motor reaches operating temperature, there is no way to shut it off, so the flow rate of coolant that flows through the hose must be kept low, or else the motor would have over heating problems when it is warm.
With the Cleveland Ford was able to allow a large amount of coolant to recirculate when the motor is cold, because the thermostat restricts (controls) the recirculation when the motor reaches operating temperature. The coolant flows through a passage beneath the brass restrictor right back into the water pump. Increasing the recirculation of coolant during warm up promotes more uniform warm up of the engine, and prevents steam pockets from developing in the heads.
If you block this passage, you have blocked the motor's only provision for recirculation during warm up, unless as you have suggested, you use the heater core as a path for coolant to recirculate from the block back into the water pump. This will work OK as long as you remember to turn the heater on each time you start the car.
On the other hand, if the Cleveland's recirculation passage is not restricted when the motor is warm, it often leads to overheating, as it diverts a generous amount of coolant away from the radiator. So in lieu of the proper thermostat, your plan of attack seems to be a good one.
George
