All the ones I have made are removable. I weld up a base consisting of a 4"x"3 flat stock and a 4" piece of square tube(I prefer square tube sliders). I then weld the mounts(4/side) to the frame, then bolt the slider onto two of the mounts.
I have also seen a clamshell style mount that goes over the frame(easier w/BL), that the sliders then bolt to.
