The cost of non matching numbers car?

[Andre]

I'm trying to establish a price on a car that is restored and received a Concours Gold award but is not a matching numbers car. The engine is correct casting and date code but not original to the car. Trans is not and I cannot confirm if rear diff is or not. Casting number is 9792977 and my research so far indicates 9783393 is correct for the differential. Is there a percentage or dollar amount to deduct? Car is a base motor automatic with factory air 1967 GTO hardtop. Any ideas?? Thanks guys.