The way to get it on/off the flash head is to put the softbox on the floor, face down, after you have it on the speed ring. Then lower the flash head into the speed ring. While I don't have a softbox THAT big, my big one (30x40 or thereabouts) is not beyond the ability of the bracket to cope with. You just have to have two hands involved when adjusting the angle. Then again, with a softbox as big as yours, I can't imagine you'd have a need to adjust the angle.
As to adjusting a boom, you just need a beefier boom. Manfrotto has a counterweighted boom arm which has geared adjustments that should handle it. If not, there are some Matthews or Avenger booms that are rigid that could probably handle the weight. If you're doing tabletop still life photos, I'd suggest getting a smaller softbox, because you're wasting a lot of your light if you use that big a softbox to light a relatively small set.