FWIW,I'd shy away from a larger diameter shaft. The larger ID bearing will have a lower load capacity most likely. The shaft didn't fail in the bearing area. The spline failed. The 4340,properly heat treated, should cure that.I'd keep the shaft as is dimensionally. No need to lose any bearing capacity. The spline area failed torsionally so there was bending involved in the failure,therefore no call for larger diameter. Matter of fact the larger diameter shaft will be torsionally stiffer,less "springy", and IMO aggravate the issue instead of improving it. Just my 2 cents.