I had this happen too and thought it was a bad dwell meter.
But, it was bad points - severely burned. The car would run but would not register dwell on the meter. New points, problem solved. Same gap setting.
The dwell can change slightly as the vac advance works, since the vac adv moves the plate and it is not exactly concentric. Pretty close though so dwell should not change much unless you have a lot of vac advance.
My experience is that if you have a good "hot" mechanical curve on the 250, say about 34 total initial+mechanical all in by 3000, then it will only want another 10 or 12 degrees of vacuum for cruise which is not very much movement. If it gets "rough" when the vac advance kicks in, you might have too much vac advance.
