Pat, the STi gaskets are going to be too thin for the phase I block. At TDC, the pistons protrude dangerously close to the thickness of the gaskets. I did this not too long ago and ended up with a knocking motor. I had to go back to OEM thickness.
I think .027" would be the thinnest I would go.....I seem to remember the pistons coming up about .010-.015" over the deck. I made a few comments about it in other threads a few months back or last winter anyway.....
Jay