Thats a rough one! The 250ex is shaft driven and the 300ex is chain driven which would make them completely different. I have never compared them side by side but I would assume that it would not work due to the difference in drive systems.
yeah, the only reason i want to try this is because i think the rear wheel hubs from the 300ex will fit on my axle. cause there is a website that i seen that says the 300ex extended wheel hubs will fit the 250ex rear axle, which makes sense cause the 300ex isn't much bigger than my 250ex. so I might have to try this when it starts to warm up and the snow is gone. right now i'm focusing on making my 250ex wider and putting some nice aluminum wheels on it, and then after that it's time to go inside the motor and get some more ponies out of it.
Well the hubs are the same I'm sure the bolt pattern is too but as far as trying to retro the brakes I don't no about that? For one the brake stop plate goes up agianst the swingarm and the 250 swing is entirely different you might can try Motorcycle Parts, Motorcycle Accessories ? BikeBandit.com to compare parts between the two and see what you come up with? Good luck