The 914 -6 GT isn't excluded because it isn't listed. The 914-6 IS listed. Engines and tons of stuff are different.
The engine in a 914-6 is the 901-03. It's the same engine found, as Greg stated, in a 69 911T. 110 whopping horsepower. Low compression, air cooled, small carbs and tight manifolds, it just won't run past 6700 or so. The 25% is a standard gain that this car will be hard pressed to hit. It can possibly hit that, but as always, it should need a full build to get there.
A 914 -6 differs, chassis-wise, from a 914-4 in a lot of significant ways. To be totally legal, it's a lot of work. Many of the items have no bearing on the competivness or performance of the car. Going half assed is an option for those who aren't concerned with outright legality, and who run in more "local" areas where such things aren't frowned on, but, anyone who does an ounce of research can spot the differences easily.
As always, it's not the concern of the ITAC whether something is easy to police or not. The book is littered with examples of easy cheats that are impossible to find, if nobody is willing to find them. As in, "Write the paper".
Gregs example of the displacement increase is as easy, or easier to discover on a 911 /914 as on any engine.
All in all, I wish it were easier to get this particular car on the track, bit it is what it is. Competitors in the NE should be on the lookout for one next spring.