Without detailed photos....
The gong makes sound by resonating and like a guitar string there are certain places you can get nice "harmonics" and other places you can get dead sounding spots.
In general, the spot at half the width and half the length of a plate is very near the "first harmonic" which is a good place. 1/4 the distances puts you at a second harmonic which is your next best choice. If I was to spitball places to put mountings to bolt a chain to, I would put them at the center of the target and then halfway from the center to the edge (primary and secondary harmonic) then use those points to angle the target when mounted.
25 yard plate I once caught a good bit of .45 Colt cast lead melded with a big chunk of gravel in the shin. It hurt through bluejeans with no other damage than a bruise but I would not have liked that big junebug sized rock to hit my face. Point is, stuff can skip back at you. Angles matter.
If you are using one hole and a bolt with a stand to mount and angle your target use a standoff so your gong is not damped by your stand- it should only touch the bolt, not the stand.. and hang freely.
The really best advice I have is "240 grain cast at 800 fps"... kabong!
