The 250r axle will not work in a 350x bearing carrier. The 250's bearing are spaced further apart. On the other hand though a 250x axle will work in a 350x bearing carrier.

On my 91 250x with 350x motor, I used a 1985 350x swingarm, shock, axle. Although I had the 250x sprocket hub pressed off of the 250x axle and installed it on the 350x axle. This helps with chain alignment problems. The reason I used the 350x axle instead of the 250x axle was because the 350x alxe is about two inches wider than the 250's and also it has 25 spline hubs like the 250r rather than the 24 spline hubs like the 250x and 300ex.