New bolt in rear end, question

[ncsurveyor]

This is my first post guys. I bought a 68 C10s back in February that I’m making plans to restore. I’m considering buying a new bolt in rear end, for one reason, just to have a new, clean entire rear end setup, and two because I’m not very mechanically inclined and I’m still learning about working on my truck and I wouldn’t know where to start as far as rebuilding. Does anyone have any experience with a full bolt in rear end? Here’s the one I’m looking at. How do I figure out what length I need? It comes in 56”, 58”, and 60”. Don’t flame me too bad if this is a stupid question, I’m trying to learn.
https://www.speedwaymotors.com/Fabri...In,383371.html

Also, what gear ratio would you recommend? I’m planning an ls swap, not sure what size yet, probably a 5.3 or 5.7. And I’m considering keeping the 3ot or maybe switching to an automatic.