hey adam.
the only real difference is the stuff around the front doors. the carpet should fit (the front part only) because i believe there is a cut in it under the front seats. the rear part wont be long enough to fill the entire door opening i dont think.
the plastic trims around that area wont fit as well.
dash will be the same. rear seats will work, mount points are PROBABLY the same.
2 dr rabbit carpet will fit and will be easier to find that jetta coupe carpet. however most rabbits you find are US built, and do not offer black carpet (black is what most people want) so you might have to look for a '79 canadian market GTI (german built) which did come with black carpet.....which, coming full circle, is harder to find than jetta coupe carpet.