JE is a tried and true piston for this application. No worries about using them. You can get them in 95mm or 98mm at 10.5:1 compression for no additional cost.

Does USC know for certain what the clearance spec should be for a Wossner in a Mahle cylinder? I'd make sure they know exactly what that number should be. The JE clearance spec for Mahle cylinders is a known 0.0015"

As a point of reference, I know of one member here who started with Wossner and then ended up using JE pistons spec'd by JB Racing. Do a search for member "gertvr" and you can see the top notch build he did for a 3.0L

If you're going with aftermarket pistons like JE or Wossner, you're selling yourself short using that 964 cam. You really want a cam (and pistons) that's designed for carbs or MFI since you're using PMOs. The tighter lobe spacing/overlap of carb/MFI-based cams is what those individual throttles can benefit from.