Actually, there is a 6.17 ratio listed for the '67-'72 C30. The axle code is MX for '67-'69, TMX for '70, TAS for '71, and RAS for '72. I won't necessarily say that it is the HO72 axle, though. I'm more inclined to beleive that it's actually the 1.5 ton axle from the trucks that used the standard pickup front end sheetmetal and IFS. I have seen a few of those trucks, but they were all badged as C40s, rather than C30. They were also GMCs, not Chevies.
