No, the Venturi was not developed from the Murena although it's true that it also started life with the 2.2 engine.
There's a very good history here
lHttp://www.pistonheads.com/doc.asp?c=103&i=3624However there was a PRV V6 development of the Murena. I had an article about it from a Spanish magazine which I had translated many years ago, but unfortunately have now lost.
The article suggested that a French company had bought the tooling from Matra to make the Murena chassis and panels and they bolted a V6 in the back.
The real problem with it was demonstrated by the picture at the top of the article - a line-up of a Venturi , an Alpine A610 and the Murena V6 which with bodywork changes looked like a cross between the two. What was the point of France building 3 mid/rear engine V6 sportscarsthat look awfully similar?
I think the built two prototypes before disappearing into history...