Niels, yes I am also on Telaio. I didn't see any of the responses except for, I think it was Marcel, who just blew it off. They might have responded directly to you or it might be because I have recently switched to the new "Geezer" list with the modern list only coming as a digest once a day.
Anyway, the engine is definitely that of the 355, hence the 5-valve stamping. Here is a picture of a 355 engine. No doubt, they are the same. The car does look like the Renault Spider, but I can find no reference to any mating of Renault Spiders and 355 motori. Very interesting.