The lower housing that was originally used on heavy duty truck applications with the 261 engine was intended to be used with an oddball "bypass valve" thermostat that hasn't been produced in decades. It had an extra threaded port for a large diameter bypass hose that kept the coolant circulating in the engine block during warmup. Once the coolant reached normal operating temp, that port was blocked by a flap valve incorporated into the stat. The diameter of the stat, the upper housing hose nipple, and the upper radiator hose was also larger than the one used on a 235 engine. Even if the correct housing can be located, the thermostat that works with it is unobtanium. If a heater core is plumbed properly, it serves the same purpose as the extra bypass hose, regardless of which size housing and stat is used, unless the flow through the heater core is blocked off by a control valve while the engine is coming up to temp.
Jerry
