There are differing opinions on using a channel section or tubes for the rear suspension bottom arms. I personally adhere to the view that DTV engineers and the original Vauxhall designers had and prefer a light channel section.
The way the rear suspension is designed to work when cornering the rear bottom arms need to twist and the channel section allows this to happen.
Preventing the twist, by fitting box or solid section, only transfers the twisting load to the bushes and suspension mounts and these resist the loads far more than the channel section arms originally would do. The effect of this is similar to fitting a really thick and stiff anti roll bar to the rear suspension. When you consider that the first mod you do to a standard Magnum is to ditch the rear anti roll bar, adding extra roll stiffness to the rear obviously isn't a great idea.
Vauxhall used channel sections for a reason. The DTV engineers did recognised the failings of the Vauxhall design and boxed in the area around the bushes to prevent the construction in that area from splitting apart under extreme competition conditions, they didn't box in the whole arm because they were aware of the need to allow it to twist.
DTV works Magnum rear suspension showing modified rear bottom arms.On the Chevette HSR the DTV gurus swapped the standard round tube bottom arms for standard Viva bottom arms with ends boxed in, to reduce the rear end roll stiffness.
I know that there are many people who defend the decision to use box section or in your case tube bottom arms but my opinion is that if Vauxhall and DTV preferred light channel section arms with stiff bushes for the reasons outlined above then that's my preference too.