I'd always try and get them off the way they were designed to come off before resorting to cuts and chisels. Remember that any serious damage to those threads will make the head useless, if you deform them with a chisel its almost impossible to get the fin nuts to go back in.
Its easy enough to make up (or have one made) the tool needed to unscrew them.
The BMW tool looks like a big open ended spanner with three studs that go into the fins.
A pair of Stilson wrenches at 90 degrees will also do the job. If you use Stilsons you need to put hardwood wedges between the fins to increase the number of fins that take the torque.
Having a 1m length of pipe over the spanner ends helps to increase the leverage, you'll probably find that they come loose quite easily with a long lever.
Use plenty of penetrating oil and try some spray CO2 to contract the fin nut as well.
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