I just installed mil-spec battery terminals and ran into this problem. Fusible link ring is too small for the 3/8 bolt. I currently have it hooked up to the thinner bolt but the mating surface isn't completely flat and the fusible link wires don't have any slack left due to being mounted further in.
What do you guys think of using an "extender" made up of 2 to 3 inches of 1/0 welding cable with 3/8 lugs on both ends? One end to hook up to the 3/8 bolt on the terminal and the other hooked up to the fusible link with a nut and bolt and then taped up to prevent accidentally shorting to ground. Would this be okay?