UM720 is the correct part. The other two are different widths and were made for the RB5 SP2 and RB5 SP2 WC kits. If you use the other braces you have to run a different suspension block and front arms.
Beware that this part bends easily. I'm not 100% sure but I think Kyosho may be revising the part soon to make it more sturdy.