You have the same bus as me, and are at the same stage.
I have 4 passenger heaters and the front heater/defroster. All four rear heaters are coming out. The flat, Romex type cable runs from the switches to the fans. The four serving the rear heaters can all come out.
The cable going back into the engine compartment has to stay, you will want it for the pump. The pump will aid defrosting when the bus is idling.
The piping ... Oh, the piping.
It has to stay ... Well, the loop from the engine to the front and back again has to stay. However, it doesn't have to stay inside the bus.
I will be removing all the pipe from the bus and re-routing it underneath, between the frame rails. I will cover it with pipe insulation to prevent coolant heat loss on its way forward.
So the heating loop will than go from the engine outlet, through the pump and on to the front heater, and back again.
I haven't actually done it yet so I don't know what obstacles I'll encounter. I was going to tackle it today, but it's raining.