this brace was loose and ran from frame rail to frame rail. But I don't know how it fits on the engine
I have never seen one quite like that, but most have the pan oil plug between the two halves. The ends fasten over the frame.
It looks like a pan support brace. The forked outer ends should grab the frame rails, as Dave said, and the brace should go under the trans/pan rear sump. Its hard to tell from your one picture but that is the first thing that comes to my mind.
I have one like it. It is a crankcase arm support. It goes under the pan at the drain plug area and hooks over the top of frame rails. They are one of the few accessories that actually do work. Do not over tighten it if you plan on using it. Tighten up till it will not rattle or easily move when tapping it with a hammer, and then tighten it little tighter.