It's been a while since I worked on a 57 but I don't think there's any hidden bolts. There should be two bolts on each side holding the bumper brackets to the frame. Probably 3/4 inch. They can be stubborn if they're the original bolts. I use my impact wrench to remove them. (the type used for changing tires.) If you don't have one or if they're rounded off you may have to cut them off. Use a chisel to split the nuts. You can't use a torch unless you remove the gas tank first.