what is up with this?

[longhorn71]

My wifes longhorn (herlonghorn71)truck is a Big-block truck according to GM of Canada's vinatage veichle services.The glove box is marked as a 396 truck.The paper work says it is a 400cid-300 hp engine?The truck is a 3 owner truck.my wife been the thrid owner,her dad the 2 owner,and a family friend bought it new.looking at the truck it has the high hump auto floor,dana rear end,big block motor stands,the big block power-steering pump in it,350 badges on the fenders and a 71 350 under the hood?We talked with the orignal owner and he said it was a 350 truck when he bought it new.So the engine must have been swaped when the truck frist got to the dealer ship when it was new.My question is what do you think the reason for this would be?Could it have been a mistake on the assembly line?