unibody
you have to grind the welds that hold the frame rail to the rest of the frame and slide the ends out . replace the frame rails and weld it back.
If the frame rails are rusting that bad the van might be unsafe to drive. You should have it checked by a qualified mechanic.
MuscleCar - 2001 Comet Frame Rails and Roll Cage was released on: USA: June 2008
From under the frame rails.
anywhere on the frame rails
Not sure what year you are working on, but yes, typically all the bolt holes will be in there in the frame. When I was a kid my dad used a 4x4 Suburban frame to make a 72 Chevy pickup a 4x4. All the holes were there, he just had to cut the excess from the frame rails.
On the frame rails.
There should be slots in the frame rails
remove the bolts at the frame rails
Try: www.interiorsupply.com/convtopcables.php
Usually towards the rear of the bus, between the frame rails.
on the frame rails behind the front skid plate