A few issues:
* A 257 Roberts reamer will not clean up the 243 chamber, as already mentioned.
* A .250" bore may not fully clean up the .243" groove diameter of the 243 barrel.
* As already suggested, you would be better off going to a cartridge based on the 308 Win if you are dead set on reboring.
* You may be better off rebarreling, it will cost about as much as reboring and re-rifling, possibly less.