Bruce Anderson has published regular market updates over the last few years that are better suited to the enthusiast-value of Porsche -- as compared to Kelly Blue Book for example. His latest update can be found at
http://www.911t.org/markupdt/update2005.pdf. So a normal '68 911's low value is about $7K and high value around $10K.
There is an interesting debate based on the engine change -- while the newer engine makes for a much much better car, should the value go up or down because the car is no longer a "true" '68?
My personal answer is that value now becomes truly personal -- how much would I agree to spend on this car in this configuration? And how much am I willing to spend to maintain it within that evaluation?
My advice -- don't worry about value until you're ready to sell it. For right now, drive the heck out of a fairly light car w/more than adequate horsepower but requiring a different skill application based on its short wheelbase.
Register for the performance driving school -- spend a lot of time with Tom Tweed.[url][/url]