Hey this is a lot of input, thanks for your IMHOīs and the links !

Guess I should give some additional info on the engine.
The engine just as the whole truck had a complete restauration and got hot tanked to clean all the cooling and oiling passages.
The radiator has been overhauled and the radiator block is new. Upper and lower radiator hoses are new. Water pump is new.

Well, there are a trans oil cooler and a engine oil cooler in front of the radiator, this may complicate the air flow.

The E-fan is a Flex-a-lite Black Magic Xtreme, thought it was the best choice in power and fit for my size of radiator. It has a big shroud that covers nearly the whole rad block, the blades rotate about half an inch from the fins. It blows in the right direction and - from my view - pulls a lot of air.

Gauges and senders are new stuff from VDO.

To list all these goodies does not ban them from misfunction ... but hey, itīs all new stuff. When there is something to supect for me it might be not enough air flow in spite of the new E-fan. I can go back to the belt-driven fan but I would like to keep my idea of this E-fan ....

But the basic question was the ported vacuum switch and itīs supposed help in reducing temperature by vary the timing advance - just for the technical solution.

And - the compression IS that high .... I bought this set of KB pistons from TMI and the advertised CR of 10.5 turned to be 10.66 in fact with 76cc heads and some milling on heads and block. Once fitted the pistons popped some thousandth over the deck, destroyed my calculation of a matching CR and gave me some serious problems with detonations.

Well anyway, thanks for your input and hoping for some more.