Just responded you to the PM, but now i see here where you are going with that. There is absolutely NO NEED for the spindle to extend below the table bed in order to achieve good 4rth axis

Meaning that the centerline of the round object could be level on with your bed. And the Z gantry clearance will be your object max radius. Add 10cm for the bit and chuck. There should be your spindle in upmost position. i recommend boxing the spindle like on mine machine and making tha gantry a bit wider in vertical. So more or less a normal machine without too much mods except for strong Z. thats it