Dana 60 gear swap

[Jonathans55]

Ok so the truck is a 55 chevy 3600 with a Dana 60 running 4.11. I have a 350sb with a th350 transmission. I want to at least go 60 to 70 without redlining almost like I basically am now. First question: which gear should I use. Secondly how ridiculously difficult is this job. I was quoted 900 to 1000 bucks for this and I'm definitely not spending that. Hell I built my engine from the ground up no way I'm paying that for someone to do it. I just like to know what I'm getting into since this will be my first time ever opening a rear end. It's like Christmas right? Lol. Also if anyone can point me in the direction for the parts Id need that would rock.