I have been promised a C type head to replace the bad one on my 3.8S 1966 Does anyone know about these and whether for sure I can use it?

 

It should go on OK, but it may not have the tapped holes at the front of the cam chambers where the cam covers bolt down near the timing chain.

 

It will fit..but you'd be better off with a correct B type head for a correct year car.. Im unsure of the C type head differences, but they were a factory 'option' offered up till about '57-'58 I think..in practice they dont add much power in comparison to a later B type head.. if seeking a 'performance upgrade' the best bet is a post sept '67 head from a 240 or 340 with its inlet manifold ( or a 420 inlet manifold) these are the 'striaghtport' head and are known to boost power output..