I've just been to hell and back with timing on a series 3 petrol. Consequently, I now know much more than I did. Other experts may disagree, but here goes:
I think 6 deg ATDC is too far retarded. The popping is unburnt fuel being combusted on its way out down the exhauste pipe. You might even burn valves in time. You need something more like TDC or even 3 deg BTDC.
Adjusting the timing may be a response to another symptom. In my case, the mixture was all to pot. First, too rich due to hole in carb float - which I fixed. Then too weak as there was a hole in the emission control diaphragm
The emission control system takes fumes from the valve rocker cover to the inlet manifold - critically downsteam of the carb. A hole in the rubber diaphragm causes air to leak in and weakening mixture. The oil in the fumes from causes the rubber to perish. Solution is to replace diaphram or remove whole emission control contraption and put bsp plug in carb as I did.
Having got this sorted, she still popped and banged when accelerating which has been sorted by replacing spark plugs.
With timing now at 3 deg or so BTDC the truck now runs like a sportscar in comparision to previous performance.