That looks like the lighter of the two Plasticote "Ford Corporate Blue" hues. Why they have two paints with the same name, I dunno. My bet is that the owner painted it with the wrong one. The darker one is a good match best I can tell.
I would contest his claims that it is rare. J-Codes are not as common as C-Codes but by no means rare. They were 302s, not 289s as the Ebay guy claims though the diff is in the connecting rods and crank for the most part. They were rated at 230hp in 68 - barely more than the A-Codes of 65-67 (225hp).
"...your son has The Knack. It's a rare condition characterized by an extreme intuition about all things mechanical and electrical ... and utter social ineptitude."
"Can he lead a normal life?"
"No. He'll be an engineer."
According to Bob Mannel's book the part number jives with the 4V. But as other posts have said it is not for a 289 as 289s came with 2Vs. C8ZE-9425-A intakes are for the 4V and C8AE-9425-B were for the 2V. I agree too that the color is the wrong shade of Ford Corporate Blue. Finally, the ad says " rare J-code." I would say they weren't all that rare as there were 38,423 J-Code cars built in 68 according to Mustang by the Numbers by Kevin Marti .
J codes are '68 only 302-4V rated at 230hp. They have a special one year only head with a 53.5 cc combustion chamber to give it 10:1 compression. Shelbys, Italias, and other special vehicles have 250hp because they have a high rise intake and revised "HiPo" style exhaust manifolds that easily clear the clutch Z bar. I can get the parts numbers off my car if needed.