As a suggestion check the needles. I once fitted a new set of needles to a set of 289's and she wouldn't lift off the slide without the choke being partially on - spitting a banging through both carbs. Comparison between the original and replacement needles showed the replacement to be fatter toward the sharp end. Substituting back the old needles solved the problem. Good luck sorting this out problem out.
I recently fitted new needles/jet tube (from Hitchcocks) which looked identical to the originals. In mid position it just kept spitting even when warm. I raised the needles a notch and it seems fine now and returning 52 mpg.
Give it a try !