I have never built an UDS, in fact I have never owned anything but the PBC (minus a mini grill for tailgating but that doesn't count). I would think that replicating the PBC on a 55 gallon barrel would provide the same results. I would assume the vent at the bottom might need to be slightly bigger to create the right proportional air flow.
I think the charcoal basket would also need to be slightly bigger in order to maintain the same level heat in a barrel with almost double the volume (I don't believe this would mean double the charcoal amount, maybe 20-25% more). I also think that you don't necessarily need 100% of the volume of the 55 gallon barrel.
Instead of using the bottom of the barrel welding a separate base maybe 12" or so from the bottom and placing the vent hole above that would provide better results. You don't have to keep the entire 55 gallon barrel hot then.
Ultimately, you are creating even more surface area if you do grate cooking, or even more space to hang meats. The problem with this entire theory is that there are so many variables that can be tinkered with it's not worth trying to replicate, at least from my perspective. I'll just stick to hanging my meat on the awesome PBC and not worrying about it until the thermostat tells me it is ready!