The negative post hitting the fender shouldn't hurt anything at all, except for possible mechanical damage to the battery itself if it hit hard enough, but if the positive hits ground, you'll know what the damage is, it will be very visible and audible
The negative (ground) is supposed to be connected to the chassis anyway, on many cars (like on my -69) there's even a cable going from the negative terminal to the fender. I don't know if that's the case on a -71, but if you don't have a cable like that, it's a good idea to install one. It will most likely make all lights in the front brighter.
If your starter is still clicking, look at the oil and charge indicators. If they light up when you turn the ignition on, and go dim when you try to crank the engine, I'm 99% sure that you have a bad ground connection somewhere. Especially check the ground cable's connection to the engine, loosen the connector, clean it up, put it back and tighten it up real good.
I know I sound like someone's dad now, but it also sounds like your battery is loose. Get the necessary parts to fasten it to the tray! I didn't care until a battery exploded in a friend's car while he was wiggling a battery in place with the cables still connected. The positive hit the fender while I was standing right next to him. He got acid and pieces of battery blown right up in his face, and even worse (according to him...) he got acid all over his -57 Bel Air. Fortunately, there was a fire hose in the garage so everything turned out relatively well (except for us, our cars and the garage being soaking wet), but I sure have a greater respect for car batteries after that deal.