I think you'll find most of the vendors sell pipe that has to be butt welded.Rather than the slip joints you mention.I used to use a similar size radiator hose clamp with a slot cut in it to hold the pipes 'square' while I tack welded them.Remove the clamp then fully weld.
It keeps everything nice and neat!
As far as going over the axle is concerned you need to allow room for full compression of the suspension and obviously clearance for the underside of bed/floor.
There should have been a heat shield on the bed wood where the original exhaust passed over the rear axle. If you still have this it can be re-used.
Good luck.
Jock
