The CE stamping is probably an original CE stamping (Not sure I've ever seen a 'faked' CE stamp)...? To be certain, you'd need to clean (with solvent) the pad and get a high resolution photo to show the factory broach marks.
From the information you supplied, it is certain possible (to probable?) that the short block you have was supplied as a factory warranty replacement into a '69 Z28, but without paperwork (warranty paperwork showing the replacement), proving it is essentially impossible. If you documented the details of the block (cast nos, cast date, good photo of the stamp), along with the details of the internal parts of the short block (1178 crank, proper rods, factory pistons) all for a 302, then you'd have a 'good story' even without the actual paperwork proof.
Was the engine in a CAR when you found it? Have you information about the CAR itself? VIN, accessories, cowl tag, etc.. heads, intake, carb, etc...