I see. When I did my research on Nardi topic and when I tried to look it up for you now, I saw posts saying 3320 fits, I saw posts saying you need to adapt connector, I saw no mention about spline not fitting.
Maybe your spline on the shaft or in the hub is damaged a bit?
I also remember I saw a Nardi catalogue listing the car models and hubs PNs, but I cannot find it now, I do not remember where I saw it.
One of the ways to find out would be to ask a Nardi dealer. I see NardiUSA page not working, I do not know what is their representation, if any, in the US.