KBB is not a good source when it comes to classic car values. You'd have to check auction like Barrett, Christies, and RM to see what those would bring. In fact, most Slant Noses you are talking about are probably outside kits. Porsche made very few cars that were actual Slantnose factory optioned cars.

This is like the difference between a Copo camaro and one that has been mocked to look like one.

I'll post more on it later. Check this for a better explaination.

http://www.qv500.com/porsche911scp6.php