Actually they DO list dually adapters for all 5 and 6 lug half tons in the old J.C. Whitney catalog I have,even for mini-trucks like Toyota's and Rangers,Vans too..the ad says to use "original steel rims " with no modification..
But yeah,I'd be nervous even with just the extra tire and rim load on the 1/2 ton's semi floating axle bearings,never mind overloading it with additional weight...I'd at least want an 8 lug 14 bolt Semi-Floater minimum,the full floater would be my first choice..then the frame will be the weakest link...
Remember the frame is beefed up considerably by the bed on a pickup,or the body on a Burb too,if still firmly attached the body shares much of the load,more so on a Burb than a truck bed,but they still add strength..
An old book I have that shows how they used to build car haulers out of ordinary 3/4 ton pickups back in the 50's and 60's ,all they did was extend the frame rails,and stitch weld a 1" solid square stock lengthwise under each rail from the cab back,and they used to haul cars weighing much more than most we drive today without too much trouble..they built them so the cars sat between both axles,so the front one took as much load as the rears,not so much on just the rear axle alone too--weight distribution is important..
