I think your old cattle panels will work just fine.
However, you might consider something solid (or with a much smaller opening) right at the top of your new screening structure.
You want to exclude all of the large and small floating debris that enters your pond after a storm. Make sure your panels stick up as high as your "flood" water level. Put some galvanized hardware cloth from the top of the panel to a few inches below the level of your hood inlet level.
After a storm, that screen will almost certainly be clogged with big floating branches (that could have clogged your outlet) as well as small floating debris like leaves and pine needles. However, your pond will drain the excess water just as efficiently through the lower 4" x 8" cattle panel openings.
Further, you don't really want the leaves and pine needles to stay in your pond. You could go out the day after the storm with a rake or pool skimmer and throw the unwanted debris over the back of the dam.
P.S. The proposed solution will be a little "ugly" in your pond, but quite effective. If that doesn't matter, then pursue your most effective "engineering" solution. However, on analogous projects on my property, my wife frequently gets a design veto, to trade off effectiveness versus how it looks. Usually a "trip back to the drawing board" can create a solution that makes ALL of the parties happy!
