I don't own a COPO and probably never will but I think your deductions are a bit high. I'm going to base the following on a price of $250K for a nice original drivetrain Yenko. In theory, if you found a real COPO from Joe Blow Chevy (-50K) with a decked block (-150K), wrong date auto trans (-75K) and wrong rearend (-10K), you would expect the seller to pay you about $30K to $40K to take the car.
You may want to study the Yenko and Copo market before you buy a car. Just my opinion.
James