1998 Ford Mustang upper radiator hose too hot

Did you bother bleeding the cooling system for air lock if not do it and see what happens. also check the water pump do below

One way to test for a bad water pump is to squeeze the upper radiator hose when the engine is hot and idling. Careful, because the hose will be HOT! If you do not feel much coolant circulating through the hose when you rev the engine, the pump may be bad. The other cause might be a bad thermostat that is not opening properly (remove and inspect the thermostat), or a clogged radiator.