Understood, are you keeping the same wheel combo or trying to go wider? Either way, you will need to cut down your 12 bolt and weld on new housing ends. Like these...
https://www.summitracing.com/parts/msr-7900
You also need new spring perches to weld to the 12 bold axle tubes like these...
https://www.summitracing.com/parts/msr-7220
Place the wheel combo you are going to use in the wheel tub, get them centered and squared. measure the mounting surface to mounting surface of the wheels. Use this measurement to know what width you need to cut down your 12 bolt. Have the housing ends welded to the cut down axle tubes. Make sure you take into account whether you are using stock drums or converting to disc. Buy new axles to fit your diff width.
Once you have the housing ends welded to the new diff assemble everything with wheels and brakes. place the diff under the car, center it and hang your new leafs. place your new spring perches between the diff and springs so you can mark where the perches need to be welded.
I would get everything set at ride height with a little weight to hold the perches in place and tack the perches in place. Then remove the diff and fully weld the perches in place and put the diff back under the car.
Needless to say, this is a quick condensed version of the what needs to be done. Make sure you account for your shock mounts and make sure you double, triple, quadruple check all measurements before welding. If you are not very comfortable with welding, have someone else do it. Your safety is riding on it.
Also, This is the perfect time to mini-tub it if you have future intentions of it. that way you build your diff to the wider wheel/tire. I know its alot of work, but thats the joy of it all.
Good luck and keep us updated
