I'm running 993TT headers on mine. Since a 993 I had to flip the flanges on one side. If I'm correct, this was not the case for 964's. The port size may be bigger, but the exhaust pipes on the 993's were only 1.5" which I don't think are to big. Again, not 100% sure of your exact application, but I'd DEFINITELY keep researching.
If 993's work, I find it hard to believe the 964's wouldn't????
Another option, call GLH tommorrow
The only reason that I know why you can't mount SSI's on a 964 is that it would choke it down. I think that if your cross section goes up in the primaries the velocity will be improved enough by design and length that the turbines will still see more heat and better pulsing. Certainly might be a good shot.