Gene:
I was looking at doing the same thing on my 87 targa. I don't profess to understand the wheel thing well, but what I did find out was that the 993 wheels will fit, but will need large spacers, in excess of 30mm on the rears.I believe the offsets on the 84-89 is 23mm.
30mm is more than Porsche recommends. You may also have some tire/wheelwell fit problems depending on the size tire you run.
Given all this, people do it all the time. I was considering still doing it, but I could'nt afford the cost of high quality spacers and found it was more affordable to go with aftermarket cups that were made to fit my car. Hope this helps!
PS - I think Leland Pate has 993's or C2 Cups on his car, you may want to ask him about the fit and check the gallery!
Take care,
MichaelW
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