Looks like the begining of a cool project...
I would leave leave the water level 10~12" below the the very top of the walls... I have a 300 gal Rubbermaid which I leave at least 6", usually a little more, below the rim... Even then I've had Oscars jump out.
Keep in mind fish usually jump out of the tank when A) they are being chased and jump as an escape route, this tank has plenty of other directions to go... or B) The tank is small, the fish is bored, curious, and wants to see what's "over there". I think a tank this size should have enough other options...
It would be great if you could could put some mature media in there to help the cycle get started. In the mean time remember there is a little under 500 gallons being used to dilute the waste... don't under estimate the power of 500 gallons of dilution... I'm assuming your not going to overstock this thing from day one...
I would say the key to success is the thickness of the walls... if the fish or the owner don't pop them... it will probably be a really fun tank... if you do pop it then downsize to the 300 gal Rubbermaid... I would think the 8' round would hold considerably more options than a 5.5' round though...