In American muscle car that's a rule of thumb
Certain cars: Hemi, 427, 454, 429CJ are as valuable as the car, and as long as its period correct...there is a minor adjustment
Now that doesn't apply to dropping a mustang engine into a Shelby!
Email Gooding, RM Auctions or Bonhams.....they have seen alot of #s
In general, its akin to having a desirable option: which has a delta of +15% in many cases.
I would disagree its a shakedown; if seller markets its as "numbers matching" then he has an obligation to back that up.
I would ask for %10 and treat yourself to a new steering wheel, Convertible top or woodkit or some other upgrade!