I have an '85 Guerciotti Pista with this exact setup and had no issues. The clearance is definitely tighter on this frame than others that I have seen with this setup, but I have had no issues.
Could it be that the BB is installed backwards? Since it is Italian threading both sides are are right hand threaded. And if I remember correctly, the Pista BB is assymetrical so I believe one side of the spindle sticks out more than the other which could be why only the NDS crank arm is hitting.