I contemplated this on my '65. I was toying with a dually step-side look, but the adapters are pricey and by the time you price 6 wheels and tires I decided it wasn't a cost effective approach for me.
You also need to check rear suspension clearance. The wheels used on duals have lots of rear offset.
All that being said if you have the bucks this can look really cool or really dopey, depends on how you pull it off.
Not sure that it's worth all the effort on a flat bed though.
My 2 cents...
Here is what a friend of mine did to his suburban... some people love it, some hate it. I think it would look better with bigger tire sidewall and smaller diameter tires. Got questions email him, he is a great guy and he makes a living doing this sort of stuff.
http://hackersparadise.biz/3_35.htmlJohn