Nick, to some degree everyone that has commented in this thread is correct, to some degree... The problem with the this discussion is that a baseline set of facts has not been determined and agreed upon prior to discussing the topic. Opinions are expressed based upon one's perception of what an LS6 is or what that means. So, if we could eliminate perception and discuss just the facts I believe that a consensus could be reached as to how it creates value. So to move the conversation along to a meaningful end the first thing that would to be done is develop the baseline facts, agree upon them, and then discuss the topic as it relates to the facts. This way everyone is talking about the same thing. For example...
1. LS6 is not a car, it is an engine option. If in 1970 you were to walk into a Chevrolet dealership and ask for a LS6 the people their would politely point you to the parts department where you could order a LS6 and in a few weeks you would take possession of an engine, not a car.
2. A LS6 engine does not create the escalated value in and of it's self. You can't take a period correct LS6 engine and put it in just any 1970 Chevelle and get 1970 Chevelle SS, LS6 optioned money. The extra value here would be that one hopes to find the original car to reunite this enigne with, or for those that want to make there documented LS6 optioned SS closer to original.
3. The LS6 engine was available in a Chevelle in 1970 if you checked the Z15 AND the OPTIONAL LS6 box. If you only checked the Z15 option box you would get the base engine, the LS5 engine. The car is the same but you had a option to outfit it with one of two engines, if made aware of the choice.
4. While the buyer ultimately determines value of anything sold, there are some industry standards that assist those in determining or assigning value. It boils down to a combination of 'boxes checked', retention of those original parts in the original car, and documentation to support the car. That's where the money lies, not just in a car, or just in an engine, or just in a piece of paper. And because the dollar amounts being paid for true, documented cars has gone up so much, the bar has also been raised in what is required in order to prove or support the car.
So, the real money lies in a combination of things that can be proven. However buyers have proven through the years that there is some in cars that have some but not all of the items discussed in item 4 above. And that is the tricky part to say the least, what is the value if you are missing one or more of those ingredients.
But to the puristist, and or the collector, the 'hole grail' of Chevelles is a 1970 Chevelle that was purchased, or built, with the Z15 option and the LS6 optioned checked, and it still retains that original LS6 engine with the partial VIN stamped on it, and documentation (i.e. build sheets, window sticker, protect-o-plate, etc.) the car and the options it included. That's top dollar. Some will say that if you have a car and an engine and the stamped VIN is the same that this is good enough. However, because of the frequency of fakes out there some may require the additional documentation in order to get 'top dollar'.
But as you can see, there is no one answer for all. It's a matter of perception and the value placed upon your perception of what constitutes value, which is more subjective. But if you're in it as an investment, the next buyer isn't buying and willing to pay top dollar for your perception but what you can prove.
However, some may argue that an LS6 optioned Z15 Chevelle value is bumped somewhat by accepted perception more so than rarity because there were actually fewer LS5 optioned Z15 Chevelles produced than the aforementioned, but, since the LS6 optioned engine was the highest rated horsepower option available in a standard production Chevelle, it derived more value than the more rare, but less powerful, LS5 optioned car.