You are too lean at cruise. That is why Chrysler’s had 3 step rods, from 273’s through 440’s. Perfect at idle, off idle and WOT. But lean at cruising speed. You can try and get it with springs or fatten up the idle, but now you are too rich at idle. Or you can get some extra rods and “modify” them with sand paper or very carefully on a grinder. I’ve heard about your problem too many times from people who know what they are talking about, some of whom where using an A/F meter in the car while driving.