I would use a small butane torch at this location. One of the "pencil" torches would be my choice. I would apply the heat right on the tip of the stud the nut is threaded onto. A soldering iron on a chunk of steel this big wouldn't do anything useful. I don't think a heat gun would either because the heat is too spread out.

2701 is the European number for what was 270 in the U.S. The same stuff that has caused a lot of problems for folks replacing Paralever pivot bearings. It is high strength and takes heat to melt and break the bond. Tom Cutter has described it as "glass like", and Darryl Mehrten has recently had Paralever pivot threads strip and ruin the swingarm when it wasn't hot enough.