Starting problem
When using the Starter Saver, there is 2 choices of retard, 10 or 20 degrees. If you were using 10 degrees on it and you have not tried 10 degrees on the new boxes, maybe over 10 degrees retard is too much?
On a 12.5 compression 496 BBC, 10 degrees works most of the time. 20 degrees can cause kick back when hot.