HW, I ran in H/Gas in the late '60's with a GMC powered '39 Chev, and you're correct that sixes should be in G or H class. Here's a link to an article that does a good job of defining class rules for the period; http://www.gassermadness.com/Gas_Classes/index.htm

You'll note in the definitions for 1965 rules it talks about stock head configuration being one of the factors. It could be with the modified head you described it bumped them into C/Gas based on displacement/weight.