I think you're looking for about a 32" long (preferrably Red) cable of AT LEAST 6 gauge.
Most of us here prefer to upgrade that important piece to a slightly larger 4ga, or even 2ga in extreme cases. But you can't go wrong with 4ga wire.
I believe they're called "Starter-to-Switch" cables, that will have the appropriate eyelet at both ends. I've noticed that many stores are stocking Black cable only now in this application, but that's acceptable too I suppose. I just prefer red for positive (and starter) and black for negative.
If you're in doubt as to the condition or age of your other two battery cables too, now's the time to swap them out and maybe upgrade them as well. A 4ga "Top Post" red battery cable about 9" to 12" long is perfect for the positive side to the starter relay/solenoid. And I typically use a 4ga black cable of (if I remember) about 42" long to run from the battery straight to the engine block to a stud near the starter.
I may be adding some length, but that's because I typically go longer so I can route them safely away from headers and other exhaust components. Clamping the starter cable to the frame is a good practice I like to follow. Again, a little overkill for most, but as I like to say "overkill is just enough" sometimes.
If you choose to use the more traditional mounting point just behind the alternator, it's probably only going to be about 30" long or so. When in doubt though, take your own measurements. As we have learned well here, every Bronco is different!
Oh, and make sure that your negative/ground cable has that second wire (usually 10ga) sticking out of the battery lug. And connect it with some additional 10ga wire to the body somewhere convenient. Your original '74 negative cable would have had a bare spot clamped to the inner wheel well for the body ground.
Have fun. Good battery cables are a life saver. Sometimes literally. Good ones, even overkill versions, are no joke.
Paul