Take this with a pinch of salt as my experience is with wood not metal but I'd say that your (white) bracing at either end is too long for that thickness of steel your using. I would shorten the bracing to roughly half way between old and new and then put stretchers between the legs (like a dining chair has). Grab a length of your 50x5 bar at the length you've drawn the brace and see if you can flex it by hand, if you can I don't think it'll be strong enough to act as a brace at that length.