Assuming a typical spindle/z-assembly the 50 x 60 x 4 isnt strong enough for that 1140mm length if its aluminium, the deflection in the middle will be around 0.1 - 0.2mm and torsionally the deflection under cutting loads at the tool tip will be similar, so needs to be steel. You'll still need to join the two box sections with a plate, or a series of plates at the ends and in the middle to transmit torsional forces from one to the other and to maintain the rail seperation accurately else you'll get binding. The other issue is that box section isnt flat enough to mount those rails on unless its ground flat after assembly, you'll not be able to maintain the rail spacing accurately enough to prevent binding. Aluminium extrusion is better but again needs to be bigger, e.g. 100 x 50 for tortional stiffness. 5mm plate is probably too thin, the deflection at full Z extension under cutting load could be ~0.2mm. 15mm plate seems to be the norm, and will limit the deflection to <0.1mm. These calcs are very rough.