There wasn't an official V6 version, but there have been more then a few 2.5 GTV6 transplants over the years.
Back when the 116 Giulietta was new, Sampie Bosman converted 2 or 3 with the blessing of a curious local AR Factory, but nothing actually came of the project. I seem to recall that they were all the 1.8 version of the 116 Giulietta as the 2.0 wasn't out yet. He turbo charged a few 1.8's as well.
We were living in Rome in '77, when the Giuliettas debuted and all my friends were gaga for one with a 2 L engine.Though the 2.5 & the 3.0 V6 engines were released in a number of Alfas later on, they would have been an obvious next-step or those inclined to take a Giulietta sedan to the next level. I seem to remember that Italian law took a dim view of anything oither than OEM equipment as all vehicles were taxed based on fuel type and engine displacement. I read somewherre that this was one reason that Alfa offered a 2.0 V6 in the later 164s.
I had no knowledge of the legacy of the Twin Spark engines, but shortly after we returned to Rome in '86, the TS 75s were released, and it was THE sedan to have. You couldn't swing a gato morte in Rome w/o hitting a TS 75, especially the beautful gray or dark blue one. I have looked as more than a few with the thought of importing one.