I'd do it in this order:
1) Spindles off (you then have the bus' weight to go against if required)
2) Old beam off
3) New beam on
4) Spindles on
I believe that you have to undo the shift rod coupler to get the beam off too. Best access is under the pedal pan at the front from memory.
Can be done in a [long]. Highly recommend two people.
If you've not done the spindles before (removal and assembly) then you might be in for some fun. If you've got a balljoint splitter you'll be laughing... somewhat.
Similarly, if the beam has never been removed, don't expect it to just drop out (support it nonetheless: safety first!) as it has been well wedged in there for 40-odd years.