Peter,
You didn't say if it was a side oiler or not.If not, you may have a 428 engine as Shelby used them in a series of the CSX 3XXX cars and then went back to the 427 dual 4 barrel engines for the end of the run.
As for it coming out of an original Cobra, that is going to be very hard to determine as it would take an excellent paper trail and they just never kept that good of records back then. Just drive the car and enjoy what you have.
Ron
