I need some help with rebuilding the rear

[AustinBelair]

I'm about to tackle the complete rebuild of my fathers truck for my mom which is a cancer survivor (For now) and I am lost on a few things.

I bought the CPP rear 5 lug axle conversion kit (That uses drums etc) but wanted to really go Disk, oh well.

I need to blow apart the stock rear diff housing to have the housing powdercoated. Once I have the housing coated I need to build it back out with all new hardware.

We are looking at a Posi and probably 3.73 gears. How do I know what Posi unit and what fitment I need (Do all GM 12 bolts translate?) How do I know what gears to get and what else will I need?

I will need to take all of it to a chassis shop and have them setup everything on a bench so I can just install the whole rolling assembly onto the frame.

OR is there a nice or fancy aftermarket Truck 12 bolt housing that will bolt in and allow me to use the correct year guts etc?

I can build motors and stuff but transmissions and rear diffs I do NOT know.