Can You make a Sure Grip?

I have put a lotta lotta street hurt on 741s and as long as you don't peg-leg it or run it outta EP oil, they'll keep going like the Energizer Bunny.
Any 8.75 Mopar SG will fit in any Mopar 8.75....... But IIRC the SGs take different side bearings than the opens. I know the track-loc does. So make sure you get the bearings with it just in case.
But for $350 he should give you the entire assembled Diff , cuz there is no demand for the rest of it. Maybe put another 10% on the table, and grab it all; save yourself the hassle of RnR-ing it.
If the ratio is different from what you currently have,that will skew your speed-O reading.