hey,
allthough i'm a little confused by the wording in your post, i'll give it the old college try. hopefully someone will add to it or correct any mistakes.
ok.
1- you have a 57 bug w/ bad channels and pans. the channels can be replaced. the pans will stay w/ the chassis when/if you remove the body, and then you can replace the pans.
2- you have an irs pan w/ no floors. no problem. buy some new pan halves. i'm pretty sure repro pans are all interchangeable up to '70ish. changes may occur in the seat rail area.
3- you want to use new pans because the bug is beyond restoration? what are you going to put on your pans?
4- modifications to the pan & body. minimal. it depends on the year of each. some tweaking may be involved.
5- go directly to airspeed and buy a disk brake kit. i believe they also carry cb performance 2\" narrowed beams w/ adjusters welded in allready. you may want to buy some drop spindles while you're at it. make sure they are disk brake spindles if you're going with disk brakes.