I haven't seen it mentioned so I'll throw it out there....could a steerable lift axle with an 8' spread of the drive axle be of any benefit?
That would give you 40,000 on the rears and 12,000 on the steers and you'd be able to maneuver a little bit better in town.
just a thought.
That is the way I had my Columbia set up for a while. It did work great, but you don't get the full 40,000 unless you spread it 10'...at 8', you get 38,000. My truck, double framed w/24' box and lift axle weighed 24,000 empty. The setup gave me 50,000 legal gross. I could carry 26,000 but not 30,000. Moving the spread to 10' wouldn't get you the needed 4,000, so you would still have to up the steer axle rating. I don't know, maybe just a regular daycab single axle with a long wheel base, add a 8' spread lift axle and a 18k steer axle? There are plenty of ways to make it happen.