Acadian's plate is a great idea, the point of using the plate, I believe, is to lock the crankshaft so that the torque you apply to the hex on the worm gear isn't "diluted". The only down side I see is that the torque you apply to the worm's hex is felt on all the rotating parts in the timing chest; not being an engineer I can't say if there would be any residual adverse effects, I would think that a new cam chain should be fitted, and I have always wondered if just torquing the cam sprocket nut induces the chain to stretch.
I can only assume that you are removing the worm gear in preparation for splitting the cases, correct me if not. I use a length of bar stock that fits through the wrist pin bores of the rods then fit a block of aluminum across the cases. when the bar stock meets the block the crank is prevented from moving. The entire torque you apply to the worm gear is now felt where it needs to act and your impact gun's hammer strikes become fully effective.
Heat the gear, "freeze" the crank and I'd be surprised if your impact gun fails to back the offending gear off the crank.
Best.