In order to have the crack stop spreading you need to drill a 1/8" hole at the very end of the crack and best to drill one at the start of the crack as well.Then the crack should be welded after its been drilled at the ends.If you don't want to go that route, and want to do it yourself...still drill the crack ends...that will stop it spreading further.Seal it with a good 2 part epoxy of some sort, and maybe a small plate over it rivited or screwed on both sides. The proper way is to have it mig drilled, and mig welded. Bondo will do nothing for the crack...coat it with a brush of primer and the plate and some sealer will outlast any bondo fix.