Locking gas cap

Not too sure where to stick this, so i'll post in general too...

Well i've got a '69 dart. It has a locking gas cap. It has a quarter full gas tank. I don't have the key... Anybody got any ideas? I don't care how mangled that cap becomes cause i have a regular one i want to replace it with. I just want it OFF! So far the most destructive way to get it off sounds most appealing cause it has frustrated me... help?