I think you're describing changing it round so that the longest axis is the Y-axis and the gantry only moves a short distance? This would make the X-axis (or whatever you want to call it) a 2000mm long unsupported length, hence the question about '20mm tooling plate and ali extrusion be enough' over this increased distance.
I guess it does mean you would only need one rotating ballnut mount. The mass of the gantry is also increased substantially.

Edit, didn't see previous post which explains it when I posted this.