surge at 50-60mph

my 70 challenger 440-6 when warm , has developed a serge when crusing at highway speeds, any suggestions would be appreciated

lean or too much advance(timming)

Probably a lean condition. What is your primary jet size?
Or your advance timing at that particular RPM.

surging is a lean condition but may i also suggest that gas seems to be getting crappier and crappier and vapor lock may be the culprit

Try ONE jet-size larger on the center-carb.

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What is your primary jet size?

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run it up to the offending rpm in your driveway & note (& post) the timing & I'm assuming this is high vacuum cruising so vac adv is all in just like when your cruising & unless total is way skewed I'd go up several sizes on pri jets. EDIT Much lesser chance but fyi if the others are good check rotor phasing; disconnect/plug vac adv (which alters phasing) and crank up the initial so the total timing is the same as it was at offending cruising rpm & see if it misses more/less or the same & if it changes holler and right now I would pump up the vac can with a Mityvac to max like it is when cruising w eng off & turn the eng slightly with the fan belt or loosen dist housing & turn it till the magnet is lined up with a tooth & see if the pickup gap is right