So looking at the chart,if my 8 1/4 is 11.69 and the 8 3/4 is 12.35 than my drive shaft would have to be shortened roughly .66 inches or between 5/8-3/4 of an inch.
That measurement came up a lot when I was doing the search.
Just to add some confusion, I had mine shortened when I switched to an 8 3/4 in my 74 Dart Sport w/904 and it measures 48 3/4" center of cap to center of cap.