273 to 340 swap in 64 cuda auto advise needed

Your 340 appears to still have it's factory TQ 4bbl. intake manifold, that's a very good manifold, and the only appreciable thing you would gain with a different manifold is lighter weight. Having the 4bbl. throttle bracket is another bonus, and for the transmission kickdown you should be able to make the one from your 273 work.