I think the chassis is born correctly. I mean well drawn, with good engineering. I do not think the 400 or so will hurt it. I think that the weak point would be rather the suspension before. Can be too weak, too soft if there is more power.
It is not so much the size of the tubes that is important. It's their use, the right order. There are plenty of examples, other vehicles MEV, lotus seven, racing cars, and the must, the maserati birdcage.