Dave,  the 3116 was exclusive to GM in medium trucks.  The 3126 came out when Ford wanted to use it.   Naturally, the larger 3126 soon pretty well replaced the 3116.

I would say that putting the 3116 and 3126 in the "same family" as the 3176/C10 and 3196/C12 is a bit of a stretch.   Make that a hell of a stretch.   About as much of a stretch as saying that the 3208 and 3306 are in the 'same family'.  In both cases they are pretty much the same age designs but that's about as far as the similarities go.