68Boss

I recently bought a 68 with a rear setup(9", 31 spline, 4.56, N case, mini spool), it's strictly a weekend cruiser. I'm looking to pull the mini spool and replace with a posi unit or possibly a locker. Am I able to just go out and buy a complete posi assembly and be good to go on installation, or what else would I need to replace(spider gears?)? Thanks for your time.