Hi Mike,
The Boss 429 is not in the FE family, it's in the 385 family along with the other 429s and 460s etc. From what I've heard it's sort of a hemi but not as much so as the 427 SOHC. I don't know the conversion, so is that like $6K US? Seems cheap if it's useable and complete. You are right it's too big for at least the ERA cobra kit without major violence. It would be cool in a falcon though
Chuck Brandt