If the disk were too thick it would not RELEASE. It would ne locked in the Engaged situation.
The controling measurement is the amout of return to clear the front disk which will clear about 8.5mm
The problem sounds more like a clutch release issue involving the slave cyl & rod. These take a different length (shorter) rod & if you bought a replacement it most likely is a generic Milano one.
As well the twin disk fits the input shafts only on the GTV6`s that they came on. Spline size & count is the same but splines are longer & cut through on the rear. Other input shafts Alfetta, Milano etc will cause the outer disk hub to bind on the rear of the splines.
Actually the clutch shop my mechanic sent it to (not the guys who rebuilt it originally) say, yes it is too thick. typical 4mm disc plus thickness of metal mounting suface. They say they have to re machine the metal surfaces or something like that (I didn't speak directly to them) and it will be the same price as a total rebuild. So now I have two dual disc setups that need rebuilding. Whats a guy to do? Well I'm having a Milano single disc clutch relined and will put the $300 savings into something else.