My simplistic approach was: Copy someone else!
Chris Cunliife has built his own M50 Cabrio, and he fitted a VW Golf header tank, and his works fine. I chose the same, and hope for the same!?!
Truth is, I do not know what pressure the cap is on the VW header - I must say I have often wondered what pressure it is, as it is plastic, not the conventional steel cap seen on most radiators.
I do know it is pressurised, as my rad boiled when I first started my engine up, as the cooling system was full of air, and did not have enough water in it. That and the fact that I had not wired my thermostatic fan up at that point. Since wiring in my 13" Pacet fan - the biggest capacity fan I could get, capable of moving 2,000 cu.ft per minute - it is pretty quick at pulling a lot of heat away from the radiator.
Mike
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