Don't have my parts book handy, but I know that the question has been raised several times by people wanting to use a 340 4 bbl manifold on a 300, and the 340 manifold is too wide. The exhaust manifolds are the same as the 300, not the iintake.
Yup! toooo wide. It won't fit. The 340 is a longer stroke, the deck height is higher (longer cylinders), so the block is wider, and the 340 intake is wider. If you want a 4 bbl for a 300, you'll need a '65 300 4 bbl intake.
Thanks, everybody! I Guess I'll just stick with my two barrel untill I find a 300 manifold that I can afford. Ben