300ex rear disc brake on a 250ex

Cable operated and i don't blame you for wanting to try this the drum brakes like to stick.

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