Yeah,

Nice fab job.

On the caster wheels; rather than sizing them up more to keep from making indents in the asphalt, I'll suggest that you just fab some take-up hardware that cause the wheels to lose contact with the ground when you're not actually moving the crane. A lot of heavy-duty welding tables utilized that method. You might even be able to use some 3" male and female pipe fittings and a circular plate that could have some kind of hex adjustment that would be heavy enough to rotate and lift each corner until the wheels no longer touch and for the best leveling, too; you know, like on a washing machine. It would probably only take about 1/2" of thread travel.