It all depends how fussy you are, intakes, carbs, headers will swap between all small blocks but how effectively the function is another story. It takes more fuel and air for a 400 engine than a 307 to start with. The cylinder heads for a 400 have steam holes drilled in them because the 400 has siamese cylinders, so cylinder heads are not just interchangeable. Another issue is that the combustion chamber on a small block head can be anywhere from 64cc to 76cc which makes lots of difference. Long story short, spend more time planning what you want to do and don't just buy stuff thinking it will work on any small block.
