How common was this particular cobra engine in the ford cars, the one that I have came out of a 1967 Galaxy 500.

I presume you mean Ford's "passenger car 428", coded as "Q" in the VIN. It was also marketed as the 7-Litre engine in 1966 and early 1967. You'll find they were poor sellers in '67 and so have become fairly rare today.

About this "Cobra" business: although most of the 1966 Cobra roadsters came through with a 427, its an open secret that some were sold with 428s instead. And this was not some kind of sneaky move to defraud unususpecting playboys of their inheritance. Shelby even published a sales brochure that plainly refers to the "428 Cobra" engine. And this brief moment in the sun is the reason why Ford still referred to it as the "Cobra" for a short time, thereafter.

Mr F