Lewis, I wish I knew how you're creating those drawings and posting them here...pretty neat. But if you look at the bold black lines which are the side walls of the tub and imagine them, say, 1/3 lower...you're now below the blue shaded area. You still haven't labeled the height measurement, but if that is drawn true to scale, (which I have a feeling it is), you should be good.
A couple suggestions. If you end up putting the keg in the blue barrel, put something under the keg so the tops of both are even. You don't want to be reaching down into the barrel to get to your actual electro tank...well I wouldn't want to. You could use a cinder block maybe..anything sturdy enough to hold the weight.
Also, consider using your white tub for a lye bath. Although you *can* use the electro to remove crud, when you see what it does to the electrolyte, (black as ink, and develops an odor), if you have the room, set up a separate lye bath. (I don't know if it came with a lid, but you should keep a cover on a lye tank). If you use your electro only to remove rust, the rust settles to the bottom and the electrolyte stays clean so you can see what the heck you're doing next time.
So I'm going to beat Cheryl to her own good advice ;)...lye for crud, electrolysis for rust. Sometimes after the crud is off, there is no rust...or very little. A couple hours of electro and you're done, vs much longer if you're zapping crud too. You'll save on electricity and wear and tear on your charger.
How close is too close? I would say less than one inch is too close, 2-4" is ok, 5 or 6 will work just fine too. It will vary depending on the piece..IMO.