Many buses have dual-filament lights for the 7" red, which function as stop/tail. However, single-filament stop lights are still out there (becoming less popular). They're $13.95 at http://commerce1.cera.net/tacbusparts/s ... cat_id=310
(second one on that page), or you can get used ones from another skoolie. LEDs are great but expensive, once I get a bus I'll be putting them in. It might not be until over the summer, but if I end up with a bus with dual-filament stop/tail lights you can have the old ones for the cost of shipping.
I like the idea of using the warning lights as a reverse lights, but I was told years ago those sealed beams aren't really designed for extended use. That would be more of an issue using them as campsite lights, but I know many here have done that also. For the couple minutes you'd use them to back up, I don't see it being a big problem. If you already have the 7" reverse lights, you should be able to splice the wires and use the same screw holes, then put the clear lens back on.