If you change it from a 67 to a 68, the vin for a 67 is in the door jamb, the vin for a 68 up is up on the dash. I have seen wierd **** with vins, and if trying to sell it later on, **** like that makes people skittish about buying. I saw a BH23 vin on a restored barracuda fastback. A BH23 is a notchback vin. Stuff like that gives people pause, especially more so if the selling price is a decent amount. So does rebuilding the car as a 68 if the title and numbers say its a 67. I know if i was looking, and saw something screwey with the vin and body i'd pass.