Running in my new cam shaft...

You did degree your new cam, right?

Is your idle speed the same as it was before? As already noted, bigger cams need more initial timing also.

Going from 252 to 256 duration @.050 will drop the vacuum a little. Doubt you'd get 19" at idle though, even in a 440. As another data point my 451 with 272 duration pulls 7" vac at 1100 rpm, and 22 initial timing...