Trust me :) the X axis dosnt hit the motor ( always thought the longest axis was the X ? .... theres a fight waiting to happen LoL ) the belts long enough to run under the gantry.
I ve seen your Z axis Jazz and did draw it up like yours but left it as it was made out of more parts and the pulleys made the router plate hang further from the gantry. I think il have to give in and sort the belt drive on the Z axis , i did think i was heading this way , with the 1:2 ratio.

Im pretty much fixed with the G540 at the moment which drives my mill at 150imp on rapids but as you say the gantry is carrying alot more weight which has to be controled when accellerating and de-accellerating while cutting.
So the next question is would you remove more material by wasting away or would you keep the mass of the side plates ... both has benifits , but i guess a lighter ganrty is the way to go
L8r