Kraig - I wouldn't blame the bolt, it's no worse for the shaft/coupler than the correct dowel pin. I'd blame a poor design! Most of the hydros I've seen in original condition have wallowed out driveshafts and couplers, with either bolts instead of the dowel pin, or a hose clamp around the coupler holding the dowel pin in place. IH corrected the problem with the Quietlines and their flex couplings on both ends of the driveshaft.
James - Go ahead and put a grade 8 bolt in the driveshaft/coupling. Be aware that this isn't a permanent fix. It will probably last a very long time, but not forever. The problem is that the hole in the driveshaft and the slots in the coupler are worn, and will continue to wear, and eventually either the driveshaft or coupler will break. The "right" thing to do is to replace both driveshaft and coupler, but that's big bucks...