Yup--there is a Pantera connection. My friend owns the very first GT5-S Pantera, the 1985 prototype. The De Tomaso connection is what drew him to the Benelli.
I believe the 750 engine is literally a reverse-engineered Honda 500-4 with two cylinders grafted on, and parts interchange between them (could be wrong on that). The 900 is a bored and stroked update.
Check out YouTube videos--some people will build retro 1960's style cafe racers with single seats, drum brakes etc. they look and sound fantastic!