[KTL]

Why the thought of changing cams, using stiffer valve springs and installing "proper" pistons? 964 cam profile and JE 9.5 pistons are not a bad combination. It's very similar to running 964 cams on 3.2 Motronic pistons (9.3 comp ratio), which is a common modification on the Carreras.

Those 9.5 pistons you have are a suitable choice for your application- carbs and single plug on pump gas.

AEM makes a nice wideband that isn't overly expensive. I have one and it's worked well for me to dial in my carbs (USA Weber 46 on a 3.2SS). You want the O2 sensor installed nearest as possible to the header collector, just downstream of it. Put a bung on both headers so you can check both banks of cylinders.