Mark, it is possible to mount to the chassis, as can be seen here:viewtopic.php?f=51&t=4609
As you can see, typically large, sturdy brackets folding around the chassis are fabricated. These are heavy and lots of work, but the approach works very well.
There are a number of suppliers making sliders such as this, and the approach works. I did something similar when I built sliders for my Y60.
However, both myself and G-Man built sliders that bolt on using the existing mounting points for the running boards, yet are strong enough to jack on. The secret is to spread the load, absorbing it straight into the body rather than trying to feed all the loads into the mounting bolts.
The main advantage of this approach is that you gain about an inch of clearance at the sills, as if you mount onto the chassis the sliders have to be removed from the body by at least 15-20mm from the body, else the body will nock onto the sliders under certain conditions, causing damage. Such sliders are also lighter.
Mag ons ons kenniskry met lekkerkry aanhoukry.