[ajmac]

Great photos chaps, always nice to see detail.
Here is my take on the PRV: Originally Flathead V8s only had one PRV, that in the valley of the block. Oil passed up from the pump then turned right just under the tappings in the top of the bell housing and went into the valley on it's way to oil the bearings etc.. (not the rear main though), pressure was controlled by the PRV at the front on the valley. When you break into the oil system before the PRV, (which you do when you utilise the bell housing tappings) that oil pressure is unregulated. When they fitted the oil cooler, the additional PRV was added to protect the cooler from bursting due to over pressure in the event of a cooler / pipe blockage and possibly as a form of cooler bypass when the oil was cold and more viscous (not on Yours Andrew, that is just for safety) However on later wartime Flatheads ford came up with a proper PRV system as part of the pump, thus oil at the bell housing is regulated.
What pump did you fit Andrew? If you have the later one you can disable the cooler PRV as you are already protected. If not then I bet your main PRV at the front on the valley is set higher than the PRV in the cooler line causing the cooler PRV to be permanently off it's seat, it shouldn't be relieving in normal operating conditions. To be honest, the pressure transmitter should be mounted on the return from the cooler to give the true engine oil pressure, I would change yours asap if I were you. As it stands at the moment your engine oil pressure could be on it's knees and you wouldn't know a thing about it!
If your cooler never gets hot, remove the aforementioned pressure transmitter and use a thin screwdriver to check that your oil gallery blanking plug is installed. When you poke through the tapping you should bottom out after 1.5" or so, if it goes in much more then oil can travel up from the pump and right into the engine, it will have no interest in investigating the cooler :-)
Ps. Any more photos your your beautiful engine?