If you are updating an MSD Analog 6 (6A, 6T, 6AL) to a digital 6 you need to be aware that the digital 6 runs on the advanced side of the pick up where the 6AL run on the retarded side of the pick up. What this means is if you plug in your digital six you will pick up almost 10 degrees in advance.
Bottom line... check your timing when changing your spark box.