Hammy
A 1972 car is getting on a bit now and is it possible that there have been some long periods of inactivity in its history? If this is the case I would be very tempted to go for a complete flush of the cooling system , drain everything right down and back flush it with a hosepipe. If this produces a large amount of deposits from inside the engine block I would repeat the process until you are happy that it is all out.
This engine can indeed be prone to air pockets , and as others have said , just keep using it , and checking the overflow tank until it settles down to a nice steady level.
Personally I would not add a pusher fan in front of the radiator as this will blank off cooling flow , the puller behind should be sufficient.
Fan belt tension used to be an issue with the early Moss Box +8, until they modified the pulleys and the belt, are you sure that there is no belt slippage.
Lastly check that the hoses are not "collapsing" under heat and pressure.
