While reading through Mopar Action, I came across these measurements. They are center of axle to center of u-joint. This should help decide on lengthening or shortening your drive shaft. This does seem to be a common question.
Axle............................ C/C dimension
7 1/4"..............................10.09"
8 3/4"..............................12.35"
8 1/4", 9 1/4"....................11.69"
9 3/4" (Dana)....................13.47"
Axle............................ C/C dimension
7 1/4"..............................10.09"
8 3/4"..............................12.35"
8 1/4", 9 1/4"....................11.69"
9 3/4" (Dana)....................13.47"