BB cars had different fuel lines, different yokes on the half shafts in the rear, different springs, different size front stabilizer bars, different radiators and no expansion tanks like the SB's had (except the L-88's and '65 BB 396 which did have expansion tanks), and a few other differences.
On the other hand, LOTS of SB cars had their hoods changed out to the BB stinger hood because people just liked them. There are a lot of cars just on this forum alone that are SB cars with the BB stinger hood on them for just that reason so if he is basing his belief that the car was originally a 427 car just because of the hood that probably won't get him too far.
Has the car ever had the body off it or the gas tank replaced? If not, the original tank sticker very well may still be on top of the tank. If so, that will tell you for sure what motor was originally in the car.