Have you thought about finding a wheel with the right width + offset?
Here's a handy calculator:
https://www.1010tires.com/Tools/Wheel-Offset-Calculator
As a reference point, the 997 Cup came with the following (EXPENSIVE) wheels:
Rear 18x12 ET30 (997.362.140.97)
Front 18x9.5 ET37 (997.362.136.96)
These dimensions would get you +4" in the rear but you would need some hefty spacers (which it looks like you already have) as the inside has 42mm less clearance.