3m makes a structural adhesive for use in panel bonding that also works real good for what you are doing also. There is a fast set and slow set version. The slow set works the best. I think the part number is 8115 Automix. Most refer to it as panel bond. it requires a special gun to use it, but you may be able to borrow one from you local auto paint store. We've used it to repair grills in the past, and it's sandable. I used it to repair the stress cracks in my header panel for my '73. Be sure you V out the crack when making your repairs.