Talk to Jason at
Turbo Engineering Aftermarket and Custom Turbochargers for Cars, Motorcyles, ATVs, All Applications and Models.
He was going to a custom GT35 swap for me, but I never made it down to Sac. He's a good guy to talk to about it. I do know they haven't done an B5 S4 as of yet, so you may be able to get a great deal.