The 5-lugs are really getting me too. Older stuff should have knock-ons, 60s cars might have 4-lugs. 5-lug to me suggests 70s-80s or later. Sure, there'd be retrofits, but on something so rare we can't place it, that seems unwise.I would almost agree with you, but I've noticed looking online that a lot of the actual Italian race cars are right hand drive, and the one in question is left hand drive. Not an expert, just an observation.
Owner said it was on a 308 chassis...I don't see any reason to doubt that. Five lugs, hip point to rear axle, prestige gap, transverse motor w/quad carbs, etc. all look spot on for that. My best guess is that someone backdated a 308 to do some sort of vintage racing showcase.