I have a 1969 c30 cab chassis and would like to know....

[franken]

If the original diff is ok, why change?
I think the 14 bolt came out in '75, but don't trust me.
There are a number of things to look at. wheel mount spacing (WMS), spring center, and shock mount. Shock mounts, or at least one are different (staggered vs on the same side of the diff). I think even the C&C in squares is going to have wider spring centers. Finally, the WMS is varied for dual wheels with different hubs, may generally be different as well. Generally diffs from a square body will be closer.
Later 14 bolts with disks will be wider.

I spent a lot of time researching this and marching around scrapyards measuring a few years ago.