I could register it,just not as an antique. Kinda like when I say a car that's the wrong color, wrong wheels, wrong interior,lowered, etc isn't restored. It's a custom still nice but not a restoration. Kansas says an antique vehicle has to be an antique vehicle, not a custom vehicle, not a hot rod etc. But if you want to do anything you want to your 57 Chevy you can, it just no longer qualifies as antique.
Also if you are registering a car from out of state they verify the frame,engine,and body number. Also if registering an Antique they inspect that. That is to recover stolen Vehicles. and so many people think it's a big deal that they do that.
But think about this situation for a minute you live in let's say Texas and have a very nice squarebody. I live in Texas and have a total pile of a Squarebody. I can steal your truck swap my dash plate and sell for a premium price in another state using my title and vin. Your police are looking for your truck. Kansas police aren't but when they find the frame and body numbers don't match they just found your truck. A car thief can make a lot more money stealing a nice car and selling it with a vin they took off a beater, than they will make parting the truck out. And parting it puts them in contact with dozens of people, dozens of people to sense something wrong and report it. Wheras 1 sale,out of state full price, done.and if it sold where they don't inspect like Kansas does, they'll get away with it.