I can tell you this - most of the types of conversions you are talking about involve attaching uprights directly to the frame, beefing up the suspension to handle the load of a second story, and building on the uprights, NOT on the body of the vehicle, as the sheetmetal will not support the weight.
You have to have an idea of what the purpose of the deck will be, the expected occupant load, whether it will be occupied or not while moving, etc... There is also the real possibility, moving the center of gravity higher on the vehiclem, it could tip or fall over. This would be uber-bad. I've seen more than one bus with a deck not being able to occupy it because the bus would fall over if moved... or if the people went to one side or the other. Outriggers on most double deckers are a huge reality on some... again, these would have to be structurally sound, tot he FRAME of the vehicle, not the chassis.
On top of all of this stuff... it has to pass muster of DMV. Just being a double decked party van isn't enough - it has to pass their Severely Mutated rules... they're even disallowing former busses that were regulars on playa becasue you can tell it's a bus. Not seeing your plans, I may be throwing stuff out you've already considered, but it's something huge to consider - I know we're talking more $$$, but it may be worthwhile to pull the van body off and build a custom-purpose body for the unit.
Also remember, once overloaded, your cooling system , electrics, etc. will have to be reworked to keep the vehicle from overheating, going dead, etc. on playa at the 5MPH limit.
But the biggie will be the internal structural memebrs you will need to support your platform, and your anti-tip technology to keep it upright with the top loaded. Above and beyond that, it's basically a decorated deck!