Can't think why they wouldn't work in a 6v system. However, I have bought a few LED stop/tail bulbs for my GT as I was fed up with it blowing conventional bulbs, all of them didn't have much difference in brightness between tail and stop. They all came from ebay and weren't E marked. I eventually fitted an LED lightboard which is much better.
I too have fitted led bulbs to both my bikes and so far so good. I have idea if the stop light is brighter than the tail lamp though I guess I will find out when I take it for it's next Mot.
But I am wondering about these led light boards mentioned above, could someone kindly enlighten me?