dimed lights and corrosion

I have a 73 duster and a 318 altenator is new, battery is tested and is good as well but I have a problem where when im sitting at a light and the lights are not bright at all and the volts look like its at 8 and when I go the lights get bright again.. also my battery leaks acid all over the engine compartment and started to rust so I took the battery out and tray sanded everything down and put some rust stuff on the bad spots and reprinted the spots but I don't want the battery to do it again any ideas