Use a 4" cut off wheel and a perminate marker to see where your cutting, ALSO PLEASE WEAR SAFETY GLASSES. Two way to consider the weld up.
We butt welded ours because there was not much pressure being put on the door check retainer area. However, if you can barrow or rent a hand held flange tool, (see link) you can off set the metal and overlap giving you a bit more area and strength.
http://www.harborfreight.com/catalogsearch/result?category=&q=flangetool