Not to but your chops too hard here because that is a nice looking car and looks to be well worth what you are asking, but the VIN will not tell you that it is an SX. All that VIN will say is that you have a 1970 Cutlass Supreme. It won't tell you that it came with a 455 or 350.
What happened to the original intake? That one is from a 73-76 455 due to the EGR port that has been blocked off. Is the rest of the engine original to the car? What does the VIN pad on the engine say in relation to the car?
Do you have a build sheet or broadcast sheet for the car? What about the protecto plate? A copy of the window sticker? Any of those could validate that it's an SX.
More information will help with the sale for sure.
Like I said it's a nice ride and good luck with the sale.