You can get linear rail material from openbuilds.com up to 150mm. A few members on this forum have extended their base machines. A common problem is the vertical spacing between the wheels is not the same as their purchased fixture and they end up with a lot of slop or binding. Suggest you also look into some sort of adjustable wheel tension system.
You new cable span is likely longer than any typical premade harness. I prefer core-xy for large frames because the motors are fixed to the frame and the only long wires feed the laser.
You also will need replacement belts. Do your current belts lie under the v-groove? If so getting the wider (stronger) belts may present a problem.
