Get all the measurements you can before you take it apart... Sometimes a scribed line where one piece overlaps the other helps, too... Big thing is patience...... Don't tack anything together until you are happy with the fit. I use Cleco's when fitting sheetmetal pieces, reason is that when you take a piece off, trim it, and do another trial fit you'll be fitting the pieces back together with the same alignment everytime... If you don't have Cleco's, a self tapping screw will work almost as well.... Clean all the pieces good inside and out, scuff with at least 80 grit (I use 36 CAREFULLY) and be sure to have all the paint off and bare metal wherever you will be welding.

If you have to use a hammer to make it fit, it doesn't fit!!!!! PATIENCE, PATIENCE, PATIENCE