I got an 15 year old 400 mast apart after watching a few u tube videos.
Basically bending the mast between a deck and the ground to get plenty of bend, in the gap add a washer and let it straighten in and work around the mast. Then increasing the washers or whatever a thicker flat piece from the garage I use.
After a while I got it apart
Another tip is use a bike inner tube to give really good grip.
Once I have a good gap and have used the file so its approx 8mm gap then I get a 100kg mate to twist.
The issue I have now is with my 460 getting stuck on a more regularly