full tubs means moving the rails in so bigger tubs can be fitted ( this being only registerable as individualy constructed!
ie: a pain in the ass)
mini tubs means widening the tubs to the rails ( this being registerable as modified production!)
It was mentioned of moving the springs inward, this could be done with consultation with an engineer, to make sure the new spring mounting locations meet satisfactory standards.( one thing to remember when moving the springs in, is that by doing this you will be raising the roll centre of gravity. this being another thing to disscuss with the engineer.)

As for buying or making the mini tubs a simple way is to mark the centre of the original tubs from front to back cut this line and un pick the inner part of the tubs. move this in the required amount ( on a 1200 ute it is less than an inch before you hit the rail ) then weld a strip the same width of this movement into the centre section.