VIN is for sure stamped in 2 places on the frame and more than likely 3 places, so using a frame that was that rust encrusted probably wouldn't be a good idea, lol
And if ONLY those 2 places were encrusted it would probably be the same as grinding them off as far as an inspector was concerned
You can get a Rebuilt or Salvaged Title in most states, when VINs don't match, and its only a $$ issue when reselling a vehicle, not for a keeper
Although it may cost a few dollars for a Roadworthy inspection to get it re-licensed with new title