getting ready to change out my 742 open rear chunk with a 489 3.91 one...on my 70 Challenger auto trans 383 car...

have read diff ways but want to hear current tips on what I need to do or look out for when changing...

any tips?

THanks Moparts...

It really is a straight forward deal. You might want to make sure the yoke is the same size before you pull it all apart. You can get the u-joint that has both sizes if you need it.
Having the speedometer pinion to match your new gear set is a plus.

It's a pretty straight forward operation. Place the car on jack stands. You don't have to jack up the front of the car, but try to get it high enough so that you have room to work. Remove driveshaft and rear wheels and brake drums. Pull axles out 6-10 inches. If you decide to remove them completely be careful not to damage the inner axle seals. Putting a roll of paper around the axle and between the axle and the seal will help prevent cutting the seal with the axle splines. Remove all but the top one or two nuts holding the third member in the carrier. Loosen the remaining nuts, but don't remove them completely. Loosen the third member by pulling up and down on the pinion yoke. Let the oil drain. Remove the remaining nuts and pull out the third member. Be careful, it's heavy. I use either a floor jack or a really big strong friend/helper. It helps to place a sheet of plywood or something similar under the rear end so that if the chunk falls off the jack you won't damage the gear teeth on your concrete floor. Installation is just the reverse. Make sure that you thoroughly clean the housing. Rags, solvent and a magnet will do a pretty good job. Clean the gasket surface and use RTV. I use a fresh gasket, others use RTV only. Install new third member and reinstall axles and set end play [ .006-.012 is good ]. VERY IMPORTANT, put fresh oil in rear end. Don't forget to use limited slip additive if new third member has a limited slip unit. Reinstall brakes and wheels. If gears are new, follow manufactures break in procedures. Good luck.

that's it. i've changed them by myself in 45 min.

Also check both cases to make sure the thurst spacer is still in them I have seen a few where the roll pin breaks after removing the axles and it falls out.