I have heard of three '78s with 455 installed by the dealer. One had been the dealer's own coach and the OE 403 suffered an infant failure of some nature and the discovered that they could get a package 455 as an install kit faster. The other was just a guy that had issues with his 403 and talked to dealer into the swap. I never got the story on the third.
The Norris built GMCs (Crestmont) were owned by Midas but built in Bean Station Tennessee at Norris Homes Inc. bout 45 miles from here. I met a feller that said he use to work there when they built the interiors for the Crestmonts and “said” that there was at least one that came thru with a 500 Cadillac engine from factory. Take it for what it is.. hearsay.. no proof but he sure seemed sincere.