289 Engine casting numbers

I have a 66 289 C code coupe. Trying to find out if it has the original engine in it. Casting numbers on the intake manifold are C 6 O, from my research that means its a fairlane intake as Z would represent a mustang. The heads are stamped 66 as well so the date adds up but i was wondering if its not a Z is there still a chance it has the original engine? The car is a late build April 66 and they had a shortage of 289 engines to my understanding. Wonder if they took engines to supply the demand. Also the engine tag says its a 67 witch im sure is just a tag that was stuck on the engine at some point as the the C6 was cast into the metal and the tag is just a tin tag.
Anyone who could shed some light on this it would be appriciated.
Shane  

there is a casting number on the bottom of the block, above the starter, which also has the date code on it, see what that says  

Will do thanks, ill let you know what I find  

With the exception of the hi-po engines, for the 289 there is no mustang specific engine castings, heads or Intakes/exhausts.  All used ford corporate engines. The best you can do is look for casting numbers within a few months of build date.