If it were me I'd use the wiring that's already there (what goes to the flashers) to power the led lightbar(s) you want to add. Why gut wire just to add more? They terminate at a flasher module (or distribution block off the flasher), so it would be real easy to move them over to your own circuit.
you could do the same with the rear (use them for speaker wires), but if your bus is wired for incandescent lights they'd probably be overkill (12awg). Still, maybe you want a sub back there
If nothing else, I don't see the point of tearing them out. Why not leave them be to power something else you might think of later?