The pressure relief valve is built into the oil pump body - I suspect you'll have to take the pump off the car.
Before doing so, is the end cover of the pump flat (smooth) or does it have indentations? If the former, it is the vane type pump and it may be worth changing it for the later rotor (gear) pump.
Unfortunately, I can't find any drawings showing the oil pressure relief valve. I can recommned however: "Rebuilding and Tuning Ford's Kent Crossflow Engine" by Peter and Valerie Wallage, published by Haynes, ISBN 1-85010-938-9 - should be available from Amazon, if only for background information - you don't have to tune your engine
Cheers,
