A 23mm carriage has a dynamic load capacity of about 6,200 lbs. So 4x would be around 25,000 lbs. A 34mm carriage is rated around 12,000.
Is the goal of replacing the cross slide to get more travel by using the rails?
Should make a nice machine. I'll be watching your progress.
Just a note: The X axis on a lathe is the cross axis, and the Z is the carriage (left & right as you are standing in front of the lathe).