I used a '73 TX750 swingarm as the basis of my monoshock swingarm. I had read that they are supposed to be a little bit (~5/16") longer; mine measured within 1/4", so I'd say the length difference is negligible.
Compared directly to the stock XS650 unit, the tubing is both thicker and slightly larger in diameter. I didn't record a weight, but as I recall it's only a very little heavier in the hand than the stock unit. Looking at it on the bike, most people wouldn't see the difference.
The increase in wall thickness and tube diameter yields a respectable calculated increase in both strength and stiffness, and as such it is an excellent swap, with or without an added brace.
I bought a Wiseguy underbrace before I became fully committed to the monoshock project... the price was right (you'd be hard to get the steel and basic tube-forming alone for what they sell for), but I was unimpressed with the fit, finish, and welds on the unit. However, looking at the basic geometry, the Wiseguy brace stands far enough away from the chainline that I doubt chain slap would be much of a problem.