Give me the serials, might be able to find differences.
One thing to check for is the VSS location, could be on the tail or case.
If its on the tail, it is an analog VSS such as in 5.0s.
If its on the case, it is an electronic VSS and the signal is not compatible with your speedo. Will need a signal converter.
The tails could be shorter or longer (different ouputshaft)
Gear ratios are not much of a problem, most of the time the difference is not that big and you can always install a different rear end gear.
The output shaft in for the 5.0 is 28 spline, just count the splines.
Tranny mount locations might also be different so its possible you need to custom make or modify your current mount or xmember location.