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...

My 68 383 Formula S came with large yokes.