actually a 300 WILL fit the early falcon and mustang engine compartments, barely but it does fit.I wonder if a Barra would even fit height wise. With it being a DOHC, it's gonna be way taller than a 170/200. The only thing I could find on it's height was from this post where the guy measured his engine and it came back at 680mm (26.7 inches).
I seem to remember a few years ago reading a post about a guy that wanted to swap in a 300 and everyone was saying it was too tall to fit. The deck height of the 300 is 10" where the 250 is 9.46" and the 200 is 7.8". If a 300 won't fit, I'd have to question if a Barra would fit. Not to mention the cost of getting one over here and all the headaches that go with doing a swap on an engine where no swap kits exist.