Not too sure what you mean about the body mounts, the sliders aren't touching the body at all,
The cab body mounts are designed to absorb vibrations, and when new usually keep the cab from moving more than 1/8" side to side, when worn, they cab can move close to a 1/4". If the clearance here is an 1/4", you should be ok.
and as far as connecting the front and rear of the sliders, are you talking about welding some tube from the slider to the step-up?
From the slider to the step up.
