Anywhere along the transom should be fine.
The easiest way is to use 2 bolts, using 3 washers and 2 lock nuts on each bolt. Make sure each bolt will be long enough to go through your transom, as well as the thickness of your anode.
Drill the mount holes in the transom, place the bolt through from the inside, with one washer on the inside. Use 5200 under this washer. Place another washer on the outside, with 5200 under it, then, install a lock nut, and tighten it up, make it as tight as possible.
At this point, you now have 2 'studs' sticking out the back of the transom. Install the anode, then, use a set of large washers, or better yet, a strip of aluminum with the holes punched for the studs to fit through, then, install the lock nuts. The strip of aluminum acts as a backing plate, in case electrolysis eats the anode around the mount holes, it won't fall off.
With the bolts placed through the hull in this manner, anytime you need to change a zinc, you simply remove the lock nuts, switch out the zinc, and put the lock nuts back on, you never have to break any sealant used around your bolts.