rearend swap questions

[Centex84c10]

Hello everyone. I'm new to the site but have been using it for most if my truck build. I'm restoring a 1984 c10 swb and have questions on the rearend. Currently there is a 1976 12 bolt being used with 3.73 gears. It has open differentials and I am wanting a limited slip. My question is should I buy a posi unit and have it installed or swap to a newer rearend. I am also wanting rear disc. I haven't been able to find an old thread to help me figure out what will work. Any and all help is appreciated.