There are a couple of different possibilities here
1. putting the brace over your existing radiator support - in this scenario you would want to tack it in around the parameter at least. The more welds you put in the more rigid/structural it becomes. That said, you will want to spot weld it or better said weld about 1" beads at various points until you have welded completely around it (assuming that is what you want to do). In other words, put a bead on the upper right corner, then put one on the lower left, then lower right, etc. This area should not develop a lot of warpage like a quarter panel would, however it is a good practice to do plus you are a inexperienced welder and doing this method makes you start, stop, reposition, repeat which is great practice all the way around. The short welds will also help keep from blowing through the material, plus it will refine your settings and technique.
2. cutting out the old radiator support and using this one as a basis for a new one. You could certainly cut off the top and bottom (part that goes under the radiator) and weld them to this new part to make a whole new radiator support. These pieces need to be welded completely along the seam (where old meets new). Use the above technique to accomplish this; tac the ends first making sure the lined up, then the middle. Make sure the seams are aligned before you weld (this is where a clean straight cut is important). Do not be afraid to grind a little, cut a little and perhaps pound a little. Do not weld too much in any one spot or it might warp. Also, do not be afraid to grind down your welds in between welds; meaning you weld a few beads, then dress them down with a grinder (do not grind too far or you will be basically where you started, no weld). This will help you approach this in an organized discernible approach.
