If you can save the sliding door track one, at least from track level and above, you will save yourself one wheel barrow full of hassle. This is quite often possible as the track will have been smeared in grease most of it's life.
The part of this sill that wraps around under the floor, with a lip on it to stop the bottom roller falling out is NOT part of the replacement panel. If you are unable to save track as above, try at least to save this part, leaving it attached to the bus floor ready to attach your new sill to. The distance from the underside of this track part to the "shelf" the bottom roller runs on is critical. too tight and the roller will bind, too loose and it'll fall out.
At the front you will notice the original had swooping bends but the replacement has hard angles. This makes them awkward to join in such a way that the door still operates smoothly.
The side opposite the sliding door is straightforward, but in both cases the inner sills run on along behind the front part of the rear wheel arch panel for about a foot.
Have fun!