Once I had the head(s) off I'd also have them checked . For flatness ,cracks et. As you remove the intake manifold look for any sign of coolant leaking at front head to manifold area. I had a time with FelPro print-o-seal gaskets leaking. Head gaskets, the blocked water passages go to the front (both sides). Coolant is pushed rearward by coolant pump through block around cylinders, Coolant then rises through open rear coolant passage in the head gaskets and starts moving forward in the heads, Coolant then circulates into the front of intake manifold to collect at thermostat area. Chrysler products use a thermostat that has a large valve as GM and will fit the large thermostat socket in Ford manifold. The OE thermostat has a large body but a small valve. Do not abandon the thermostat bypass below the thermostat from block to coolant pump, otherwise coolant stops circulating in the engine completly untill the thermostat opens. A small hole up high point in thermostat helps get the air out of engine coolant system especially when refilling coolant. Also makes sure that the thermostat is in the flow of the hottest coolant so it will open and send that hot coolant to the radiator.
My brother in law had a 84 F250 Coolant recovery tank all ways empty and
antifreeze all over the place. Turns out the thermostat bypass hose between intake manifold and top of coolant pump had an internal blister. It completely shut off coolant flow untill enough hot coolant at thermostat collected and then the thermostat would open violently. Coolant temps at rear of the engine were even higher causing really hot coolant to rush into radiator and then opening the pressure cap release valve to discharge hot coolant violently into the coolant recovery tank.
I'm using the old style surge tank with a closed system cap and then a coolant recovery tank to collect expanded hot coolant being pushed out of engine system when up to temp and then refill it's self when cooling.