From what I have heard once you get the crank arm red hot they bend easily. I will also be bending up a set of cranks for my bike to clear the engine.
I have one of these Weed Burner Torches that seems to produce a hotter flame because of the type of burner tip and design that introduces more air into the gas to air burn ratio.Using a MAPP gas cylinder on your torch will get the crank red hot, but it'll take a while.
MAPP gas burns considerably hotter than Propane.