Yes, the carriage height in Z (top to bottom of the moving body) is very short and quite a bit less than the overhang of the spindle. Also the depth in Y from the spindle to the column is fairly limited. Once you have a vise in there, you will lose some of that, reducing it even further. The Sieg cast columns are standing on spindly pillars - solid blocks would be sensible.

Looks like a plan though - get cracking and show us how you get on! You are bound to evolve the design as you go so the sooner you start the better.