I have been chasing the same problem on my stroked 440 for 3 summers.
i have finnaly determined the the the problem was the carb .
it was to lean in the transisiton circuit of the carb .
This is where you run most of the time when crusing, changing main jets only masked the problem and makes the engine run to rich.
For you you may want to look into a couple of theads , Tunning with an o2sensor . and Demon carb ?'s . There is good info about tunning the idle feed restrictions to get the car to cruse at the proper a/f ratio required by the engine . every motor needs its own a/f number keep that in mind . hope this helps you . CJ