Before drilling and tapping the casehead, loosen off the decapping stem, and get it as far from the head as possible.
Drill and tap the casehead to 1/4x20.
Find a socket that is larger than the casehead, smaller than the die. Run a 1/4" bolt with washer through the die and screw it into the case. Pull the case loose by tightening the bolt. You may have to adjust the length of the bolt.
Disassemble.
Saw off the case head.
Remove the decapping stem, and the front portion of the case.
Clean, reassemble, and use more lube - not too much, though, or you will dimple your brass.