Another "Can of Worms" thread LOL
To answer the OP's question my definition of #'s matching is as follows:
Numbers Matching has 3 levels
1) Dash Vin matching stamped body #'s, engine and trans #'s
2) Components listed on build sheet match IE rad, shroud, fan, wheels, T bars etc
3) Then Date Codes of various parts
More $$$ as you move 1-3
I don't know what the legality is when you sell a car where the vin indicates the car is a Slant6 auto and the car was changed to a 440 4speed or when the car had drum brakes but they were changed to discs or.......
It's your car, do what you want to it.
Is a 68 6 cyl Barracuda car changed to a 340 Formula S worth the same as a true #'s matching Formula S I think not.
What if the seller doesn't disclose this info?
Let the buyer beware