OK, Jim, I think I better understand your situation.
Under the current rules - through the end of this calendar year - the rules stipulate that the chassis/VIN/drivetrain must all match. In other words, in order to build a 1.6L Rabbit, you must build using a chassis/VIN that came with a 1.6L engine. Sandro is correct in that regard.
However, from 1/09 forward, that "VIN rule" is rescinded. At that point, you "may" use a chassis to build a car but not have the VIN match; HOWEVER, you cannot create a model which didn't exist before.
Example: the ITB Rabbit GTi had an engine VIN code of "D" (was it?). Ergo, currently it would be illegal to take a 1983 1.7L ("C"?) Rabbit chassis, install all the GTI gear, and run it in ITB as a GTI, even though it is, in all functionality, the same car as one originally delivered as a GTI. Why? Solely because the VIN is incorrect and doesn't match the running gear.
However, effective 01/09 it *would* be legal to do so.
You, unfortunately, are in a tough situation. As far as I know, all US-built, large-taillight Rabbits were either 1.7L or 1.8L cars. To cpmete in ITC with your 1.6L running gear, through 12/08 you must use a German, 1.6L VIN chassis. However, from 01/09 forward, to race legally in ITC with a 1.6L, you *must* build using a smaller-taillight, German-built chassis *unless* you use a US-built, larger-taillight chassis as a baseline *and* convert all parts that are different to German specs.
Yes, it can get confusing, but if you keep in mind the cardinal rule* then you know what you have to do.
Greg
*The post-VIN cardinal rule: You cannot create a model which didn't exist before, in each and every detail.