OK, more questions.. do all the ground wires including the ground wire from the main panel and the ground wire from the local ground rod go to the ground buss?
You addressed adding the floating (not attached to the box or the ground buss.) neutral buss, and I assume the neutral from the main panel feed goes there as well as all the white/neutral wires in this building and DOES NOT connect or bond with the ground wires.
You just failed to address what exactly happens to the ground wires on the circuits and the ground wire from the main panel and the ground wire from the new locally driven ground rod.
And from what I could see of his box, he does not have enough space to have a 30 amp disconnect to accept the feed from the main panel AND also have room for another 30 amp breaker for the welder.