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I think the first thing is you measure the backlash at maybe 4 points equidistanct around the ring gear & note the tightest (smallest) dimention & record that number. The used pinion bearing that is behind the beveled gear teeth would need to be pressed off & slightly ream the ID so it will barely slip on/off the new pinion shaft till you get the right shim pack thickness figured out that goes between the bearing & the gear teeth flat to get the pattern right. I'm in the same boat & there some internet sites that have procedures. I'd try Randys ring & pinion for a start. the ring gear bolts are left hand & you want to deburr the new ring gear with a whetstone so it sets dead flat when bolted on. The 489 may not use shims & you just torque the pinion nut & the solid spacer has been highly recommended rather than the OE crush sleeve. Here's a BTT for ya & others will guide you better
Read (and understand all the terms as well as the procedures ) the section on how to prepare that third member and the tools needed( pinion depth seting tool. dial indicator with a good magnetic base, inch lb torque wrench, 250 Ft lbs torque wrench, proper size spanner wrench) and so on to set those type gears( 1 7/8 inch tapered pinion gears with a crush sleeve) up correctly, if you want to continue then buy all the tools needed and have fun 
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Before you remove the third member from your car go buy a factory service manual for the car, all Mopar cars that came with 8 3/4 stareted using the "489" third mebers in them so make sure the manual you purchase covers those years from 1969 to 1974
Quote:think twice, act once
I am thinking it would be better maybe for me to have a shop do it that sets these up to make sure cause i don't want to get it wrong for sure.Quote:
Before you remove the third member from your car go buy a factory service manual for the car, all Mopar cars that came with 8 3/4 stareted using the "489" third mebers in them so make sure the manual you purchase covers those years from 1969 to 1974
