Rear end Toast!What Fits?

[tjm52]

Hi! I really want to keep my 65 C10 a 6-bolt, but I have discovered that the diff is shot with a capital S. Both axles and bearings,crown and pinion are badly pitted,spider gears the same,all the internal bearings too...The only thing any good is the housing and I really cant afford to replace all the insides. 64-72 coil susp. only? Can I rob from a newer 12 bolt for the center section and go find some used axles? Is there a 3.5ish ratio around so I can cruise safely around 75? 250,mild cam,headers,stock intake.Just learning! Tom