A million inches of travel, Hmmm... that sounds like it would be a nauseating ride.
I haven’t seen much on Bus beams and brakes here but you will have to figure out how to dumb down the front braking system down. Since rails don’t have much weight up front you will have some work to do getting the rear brakes to be the major part of the system; e.g., getting them. to engage first and get them to be the part that does most of the work. You might need an adjustable proportioning valve too.
Usually the slave (wheel) cylinders are replaced by small diameter ones usually out of the rear (sedan) but I am not sure how they fit in the bus backing plates. I would go to a sedan dual chambered master cylinder (M/C).
Removing splines and epoxying fill plugs in their places should work.
Figuring out and hanging the shocks should be fairly easy. Stops will be needed and maybe you will want to keep the sway bar.
This will be an interesting build to watch. I hope you start a string and keep us posted (no pun intended) on what you come up with.
A couple books out there they you might want to keep an eye out for. Earl Duty’s; How To Build A Dune Buggy, An Illustrated, Step-By-Step Guide to Building A Dune Buggy This book deals with building a street driven rail. It has a lot of good information for just what you are trying to do… with the exception of the Bus stuff. You might be able to find it at your library before you go out and buy the book. Another book for your library is: Baja Bugs & Buggies. The author is Jeff Hibbard. Granted it is more about Baja’s, but still there is a lot of other good information there.