They usually put a lift axle on a double frame. I have replaced and repaired frames double and triple, at the lift axle mounts, due too stresses. Your box looks to be a rigid mount, if you drop the axle and pump significant air, that frame will flex in the centre. Which will transmit to the box ?
The frame on the truck appears to be a regular highway truck frame 1/4 inch wall?? I've repaired Mack triple frame at the lift axle mount, that's just over a inch thick.
The build looks great,mu doing the interior wood working too. I can't get my welder to stick two pieces of wood together yet.
Here is a little Rv hauler I am doing right now, jus finished putting it into primer tonite