The bracket goes on the top thru bolt on the starter, the one that looks like a stud with a nut and washer. I haven't seen one of these intact in years, so all I can say is to mount it to that bolt/stud on the starter, hold it in place (or bolt it up to the mount so you don't have to hold it, I know they're heavy!)and see where the other end lands. The bracket's curvature should d follow the side of the block.
I'm not sure the 400 I have, still has a starter on it, and that's the only small block I have that's old enough to have this bracket. These were a great idea, but many got tossed when the starter was changed, because "it's in the way, what do we need this for". GM wouldn't have put it there if it wasn't needed.
