I’ve been traveling and not been able to type.
Shaft measurements are Center Eye to Center Eye of U-Joint.
That distance is not easy to figure out (easily).
If propeller shaft has same U-Joints on both end (and should have, unless a custom shaft), the
trick is to measure from U-Joint cap to U-Joint cap.
Cap one is easy to hook a tape measurer on.
Cap two gives a clear concise line at the beginning of the cap.
This shaft is from an ‘84 Gran Fury Police (M-body) 904 and 8-1/4" differential.
It helps to have two people with this, for tape measure end has a tendency to wonder, but not required. Hold one end of tape to cap end.
Measure from the Red line, and not the Blue Line
I had to rob the slip yoke from this shaft - so is missing on my shaft. Line up tape to same end of second U-Joint.
In this case, this shaft measures 52-3/4" (which DOES NOT apply to your J-body).