Very few dealers did this and even fewer are documented.
Its a unicorn, go luck on your search for more information
+1. many conversions to the 427 but they were typically done by previous owners and not dealers it was not a easy task back when the 67 Shelby's sold new. were new . The swap required headers . No ready made headers for 427 heads in a Mustang body at the beginning of 1967 production. Headers had to be custom made . There were other things that had to be done for the conversion also. Consequently it would be not only expensive labor and parts wise but a daunting task for the dealer to figure out how to do it and what was needed.
IF the 427 was a Ford Dealer installation, the engine installed would likely be what was available as a service part at that moment.
There were 67 427's listed as available for a short time in the '67 model year but the quantity must have been limited and they did not last long.
I'd take a guess and say that it is more likely a '68 W code 427 and that would help with the exhaust manifold situation not requiring headers.
Now if we found that there was a 427 still installed then with the story of a Dealer Installed engine, then at least the casting dates on that engine would come into play as making it a possibility instead of another "my dog ate my homework" explanation again.
No one is going to buy that story if you find it is a '63 center oiler or even a center oiler at all since a new service assembly would have been a side oiler. The '67 version is the "good one" and probably the scarcest of the lot to find intact.
So far the only documented '67 427's are the Shelby installed ones.
Dam eternally that first Registry "Dealer Installed 427". Who came up with that place holder moniker? I suppose that sounded harmless enough at the time?