489,741 or 742, the Difference?

The axle spline count (30 spline) is the same on all factory 8-3/4's. The driveshaft from your 7-1/4 will not work with the 8-3/4. Different length. There are a few different yokes. I believe all A-bodies used the small 7260 yoke. You could find the small yoke in some other body styles also but it's hit or miss.

Here's a good article comparing several different differential pinions

http://www.doctordiff.com/blog/tech-info/rearend-pinion-comparison/