I'm of the age that I had done much mechanic work (including racing) with the 312cid engines - and sludge build up in the rad was common. I do recall changing several sets of head gaskets from water seepage - and usually what you end up with is milky/cloudy looking oil and sludge. It isn't usually really bad enough to notice except for those two indicators. When you finally pull the heads, you usually only see a dark band where water and/or exhaust gases have passed - and those that I saw, it usually, and it might only be coincidence, wa between cyls 7 and 8 (left side rear)

Oh yeah - these engines have tender main webs, 5K rpm and higher