Your strength in your frame is in the flange, and thats where it is cracked. If you are actually use the truck, it needs to be repaired. I would have a glove bent to cover about 2 foot of frame. I would remove the rivets and replace them with flange bolts. I have been altering frames for 50 years and this is how we do it. If you are just going to casually use it, you can v out the crack, weld it, and weld an angle iron inside of the frame to support the weld.
