or you could just replace the synchros and bearings and run it for another 500k+... it may be the best option anyway considering that buying a used gearbox you still dont know what your getting, how it was treated, whether the bearings/synchros are good,etc until you install it and go for a drive. then if its not perfect you are stuck with 2 screwed up gearboxes and the money you spent on the replacement could have been spent on rebuilding the original. not to mention the labor removing and installing them multiple times.