Not a dumb Q at all, I learn here every day. the heat riser is the flapper valve on the outlet end of the pass ex manifold but w you having a blocked gasket that would take care of most of that potential heat source. When you start it how soon is it acting up? What do your plugs show? Water flows into the lower holes in the front of the block and flows around and to each cyl from front to rear and goes up into the heads via the holes above and below the cyls then circulates and exits the head(s) at the front of the heads into the large hole in the front block deck & a short turn to the 2 large top horizontle holes at the front of the block & into the pump housing. If you pull the heads I'd block the large hole at the rear of each block deck and drill a 1/2" hole in the deck and heads between cyls 2 & 3 between the head bolt holes (& punch a hole in the gasket) as this is the hottest area w 2 ex valves being paired side by side but I dont want to encourage you to do that at the moment as there is something else going on here