Further investigation revealed Wedge Top Pistons had been fitted ..... with the wedge to the Inlet valve side of the engine .... really .... this effectively creating two combustion chambers as the spark plug is positioned above the Exhaust Valve. If that wasn't bad enough it would also have the effect of trapping exhaust gases under the Inlet Valve as the Exhaust closes for the Induction phase.
A redesigned set of Forged Pistons were ordered from JE Pistons in the USA - Awesome service - manufactured and delivered within 14 Days.
The Engine had a tendency to run very hot, and the Piston vs Combustion Chamber lay out would not have helped this causing a slow burn, possibly exposing the cylinder wall to a still burning fuel mixture elevating the water temp as functioned more like a wetback water heater.
The new Piston tops would sneak up the compression ratio ever so slightly creating some swirl and flow towards the Ignition source, better combining the air fuel mixture in a relatively low compression engine for today's pump gas. The Wedge Top would also aid in evacuation Exhaust Gasses on the Exhaust Stroke.
Remember the offset crank shaft - at the time of combustion the crank has already passed no-mans land at the top of the stroke so the piston falls away very quickly - again a slow burning or slow to ignite mixture may still be burning on the power stroke elevating Cylinder Block water jacket temperatures