There’s a long thread about belts and belt stretch. The Carbide belts are now, I believe, steel cored, or there’s the kevlar cored belts from RepRap.nl which I’ve installed, this is sold by the meter so you can run any length you think will work.
The Carbide extrusion is one of the over-engineered parts, if you can get a longer section then it is probably still not going to be the weak point, you could always fill it with epoxy granite if you wanted to damp out the vibrations (more important over those distances).
A machine that size is really going to need a rigid base, not to be hanging off it’s own transit rails. You’ll probably gain the most by welding a steel box section base or building a large plywood torsion box to attach the machine to rigidly. There’s quite a few threads here about pimping up the machine. At that size you’re not going to be able to use any of the chassis front-back rails or stock wasteboard assembly so you might as well build something more rigid to start from. This is why the Avid machines have that large extrusion chassis and the welded steel leg frames. If you can’t hold the work square, level and rigid then it doesn’t matter what your gantry is doing.
HTH