As Tom says, you can only determine whether it was originally a 400 car is through PHS. The VIN cannot tell you that it was a 400 car.
If however the engine has a partial VIN that matches the car VIN (last six digits) then the original hood was probably damaged at some point. Look at the underside of the hood and see if the paint matches the body color in the hidden areas of the hood.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
