To OP: nice piece of work, occasionally I wish I had a welder I could pull out and knew how to use it. :)
Your spare tire reminded me of a discussion that took place last year at the annual b-rally.org Class B Rally about whether a spare tire is really necessary (especially since it was said that several current Class Bs are sold without them). The spare tire on our 2005 LTV was on the back so LTV could create a "basement storage" like you have created in place of the spare tire (the generator on the Sprinter chassis is in front of the axle). We now have 153K miles on our LTV and the only flat tire we have had happened when we were parked at a relative's place overnight and the tire went down due to a nail picked up somewhere that day. This episode educated us on the difficulty of changing a flat tire on a Sprinter van. We got it off and the spare on only because our host had a heavy duty lug wrench that allowed us to get the lug nuts off, the lug wrench that came with the Sprinter was useless. So anyway, I took our spare off almost a year ago and it is stored away in our garage based on our almost no use of it and the growing aggravation of it being in way of the back door (especially after putting a screen in the back openings. I figure I can use our insurance that covers roadside service if we ever have another flat tire, unless we decide to go to Alaska again (unlikely).
But to get back to the "basement storage", when we were shopping for a Class B back in 2005 the "basement storage" was one of the three things that sold us on the LTV vs a couple of other Sprinter-based Class Bs that we liked. I think you will find it as a helpful addition to your Class B. We certainly have.