Not all 6wd trains are cantilevered... nor do all of them use hex shafts. Keyed round stock has served us well for years.
Here's a shot of one side of our 8wd system from this year... but if you just lengthen the spacing between the wheels, you've got a 6wd.
The centre wheel in this shot is mounted directly to an extended output shaft from the KoP toughbox. (Geared down to 14:1)
Tension is adjusted by turning down the nylon rollers until they are "just right".
Jason