If it were a J code Shelby it would be less of a dollar amount, but I think as a general rule of thumb Jeff is correct with the percentage range given. A '68 GT500 would be a bigger amount of cash, and a GT500KR an even larger pile. And we have to have two identical color cars, with identical options, restored to an identical level of perfection.
In the real world all those things are unlikely to be equal. One guy might overlook the non - matching engine if he just had to have a certain set of options or a certain color combination. A different guy might not even consider buying a car that was not numbers matching, under any circumstances. So the dollar value is one thing, and the ability to sell the car is another thing. A matching numbers car will always hold at least one advantage over a non - matching one, or many advantages.
Like most things related to the value or selling of these cars - it depends on the potential buyer. For some a car without the original engine is a non-starter while others could care less. So your left with the group that cares about dates and details but are not completely turned off or bothered by the lack of original engine but has a period correct engine and are willing to consider it for a discount from a matching numbers original example for sale elsewhere. Since you did provide that the car in question is a restored 68 Shelby that removes some of the other components of the deal and narrows our scope.
As a non-regular buyer of these cars in more recent years I would guess that you might be in the 15-25% area. Sure others will have more informed opinions. Maybe the 68 guys (like Pete or Tim) will have a more focused response and experiences.
