40W and even 50W should be fine. Some low quality 40W heaters have all the heat out near the wires and none at the tip of the heater but the ones made by Ultimaker and 3dsolex should all be fine. Brass melts around 900C! There's no way you can have the back of the block at 900C and the temp sensor as cool as 210C, lol. Long before it hits 900C it will start glowing red hot and you will wonder where that "red light" is coming from.
Now if your temp sensor falls out but the heater is still in the block, and if you have the older firmware that doesn't report "heater error" then, yes, I suppose you could melt the block.
I am not sure about the 35 watt heater but the 40 watt heated melts some of the block on my friends printer.
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