Alex,
I did some research on ally unprights a while back. There are a few who claim to be able to supply, but the only people I spoke to that actually had the items in stock were Ray Christopher & GD sportscars.
Ray makes rear ally uprights for GTD's, altho it's not a 'bolt-on' & not particularly lightweight. However they look the biz (very original) & are considered very strong. He also sells rose jointed suspension arms/links which you may well need.
GD sporscars can supply ally uprights front and rear. The items are well proven and used on their cobras & T70 rep. As they are not designed for a 40, new wishbones have to be fabricated and/or the uprights machined. This is pretty specialist work & needs lots of care & skill.
MDA has ally uprights front & rear on his new GT40 kit so there may be an option but you would need to speak to him direct.
Flex in the uprights is probably more due to tyre choice than brakes.
Hope this helps. /ubbthreads/images/graemlins/smile.gif