Howdy there again Marpar! I immediately thought of you when I came upon a story in the October 2017 issue of Mopar Muscle. They did a nice little story on the slant six called " Slant Sickness". The give some history and try some dyno stuff on it with different manifolds. I forgot about the 390 four bbl. carb as I thought it was a 345 or something, warped memory. Anyway, it might help you out some in your quest for information and save you some headaches! I hope it does, best of luck with it! 
