Do the bottom with the plasma cutter. 10"-12" diameter is best.....10" lids are easiest to find.
If you leave the sanke coupling at the top intact, you've got yourself a 2" triclover bottom drain when you invert it. This is only if you're going electric. Otherwise, cut the standard way as others have described, but keep the spear and newly cut out top. Some people weld fender washers to it and use it as a lid.