I have a 96 F150 4x4 with the original 351 Windsor
A label on the valve cover says " 6K408AA.
Is there a factory serial number stamped on these engines, if so can anyone advise exact location.
Thanks for any help.

 

Of course there is. It's on the back of the #8 cylinder, just above the bellhousing. But don't expect to be able to read it. It's pin-printed through the paint, so at this age, it'll be rusty, like this one:

(click this text)