If you are doing electric arc welding you often use an inert "cover gas" to keep atmospheric oxygen (and perhaps nitrogen) away from the weld. This gas is usually heavier than air and so falls downward to cover the active welding zone. If you try welding upwards then the cover gas falls away from the weld and you don't get the desired effect. This means that you need to arrange the work area so that the work pieces are suitably placed below the welder to allow the cover gas to do its work of excluding air.Funnily enough a week or two back, Tank Museum put up a pic on their Instagram feed of an assembly rig for Centurion. Because manufacture required lots of welding at just the right angle, rather than have the welder moving all over the hull, the rig rotated and the welder got every seam to be welded presented right in front of him (her in this photo) at just the right height and angle. See if I can find and post. Wait out.