Incorrect chassis number will always raise question, a police number would mean I would just walk away from it and never return.
As for grinding off chassis numbers, well that is way out of the realms of what I consider legit. I have two blank RTS chassis here, and considered a few options with them, but in the end the only person I would be fooling was myself.
Just find the right vehicle to start with, thats how you avoid the hassels.