The only thing I can suggest (and I may be way off here since I have no flat head experience) is try to wire an electric fan on there and see if that makes a difference. That, or try to bleed your cooling system and make sure all the air is out of it. You'd be shocked how many people forget to do this.
The radiator cap in my avatar racecar is slightly lower than the thermostat housing (six cylinder Holden Grey engine) and when I fill the radiator I jack the front up so the cap is higher so all the air is purged or I remove the temp. sender from the thermo housing to purge the air.
The Jalopy Journal
