I have used nuts and bolts for hinges. They will seize from rust, otherwise it works.
1/2" pipe and 1/2" bolts for king pins and spindles or tie rods.
You could drill the threads out of a 3/4 nut and use a piece of round stock. Weld the nuts to the frame.
Most nuts have zinc on them, don't breathe the white smoke.
For the deck 11 gauge is plenty I use 10 gauge in small dump trucks. You can also use wood with a tubing subframe.
The bigger the wheels, the easier it will pull or push.
Looks like a fun project.
2012 R1200R !
2000 R1100RT (parts for sale!), 1976 R75/6,
There is a seat for everyone.