Thanks Mike, I can't find my numbers book and was going on the Prestolite info. Now it begins to make sense. The specs for that distributor are 28 degrees at full centrifugal advance. Plus vacuum advance of 21.5 degrees at 15 inches of vacuum. The 1970 318 is supposed to be timed at TDC. Start there, I'd also be suspect of the aftermarket Holley carb jetting. I'm assuming Champion N14Y plugs or equivalent. 1968 is the changeover from closed chambered heads to open chambered heads. Who knows, maybe the higher pistons with the leftover closed chambered heads made for a higher compression motor? I know my brothers 67 Barracuda 273 2 barrel short block would ping on 89 octane fuel with closed chambered heads and the big Fel-Pro head gaskets.