From Greg Field's "Moto Guzzi Big Twins":
Individually, Carcano and Todero had each woked on 90-degree Vs in the past. Carcano had designed an air-cooled 600-cc designed to propel the Fiat 500 car, and Todero had helped develop the fan-cooled 750-cc engine for the Mechanical Mule, an engine designed by another Moto Guzzi engineer, Antonio Micucci.
Neither the Fiat 500 nor the Mule engine was used as the basis for the V700 engine, however, and it's somewhat puzzling how the whole misconception got started, given that the Mule engine was designed Micucci, rather than Carcano. The fact is, all the V700 engine and the other two share in common is a 90-degree V-angle. "Comparing the head design and structure, the distribution, displacement, lube and fuel systems, and engine crankcases is enough to demonstrate that the engines are totally different and come from different ideas," asserted Todero.
I think the above says it all. Similar, but completely different.
