I don't think you will gain much going to an early 60's rear end. It probably has a 3:90 ratio & yours is probably 4:10. I don't know what engine you have but some of the 67-72 c10's had 3:08. They may be to wide for your 55 & i'm not sure the original engine will pull it.
What about changing to the the gears in yours to the 3;55 gear set?
George
