Check out this thread; it’s the best aluminum milling I’ve come across using the X-Carve.
The height of the block of aluminum shouldn’t be so much of an issue if your part geometry allows for the spindle to be lowered close to the cutting surface as it works. The support plates for the y-axis have been modified to allow for a greater z-axis travel.
I can’t imagine there is any modification you can do in order to use the X-Carve to cut steel in any serious way. The differences between steel and aluminum milling are too great. Typically, rigidity (stiffness), powerful spindles and drive motors are what more expensive machines have that allow them to mill steel successfully.