I can see both sides of it.

My take on it was I was not allowed to mix and match parts to make something that didn't exist. My original question is on the Mustang and convertibles, so I'll use that car as my example.

94-98 Mustangs are all on the same spec line.

However, while gross specs like horsepower, wheelbase, brakes, etc. didn't change during those years some other little things did. 94-95 cars have a different rod ratio than 96-98 cars. 96-98 cars have changed with heads and blocks/block sealing than 94-95 cars. 96-98 cars used a slightly different valvetrain parameter. 94-95 cars do not have the same cam as the 96-98 cars.

So I could not build a motor like this:

*94-95 crank and rods
*96-98 block
*96-98 heads/valve train
*94-95 cam

as Ford never delivered such an engine in a Mustang during the 94-98 model run.