Other thing I failed to suggest: have radiator pressure and flow tested. It may need a real pro boilout and rod out. It's like plaque buildup. Too much and no flow=no cooling. I'm still more concerned about the same condition in your engine block. That's why I suggested the two part caustic. Hell throw an old core water pump on there just for the flush if you are concerned. Actually the ones that fail the flush are old worn out ones where the packing was wore out and there was just gunk plugging the seal. Gas tanks and radiators will make or break you. Verify these are good before poking around elsewhere.
