Tough one!! With all those imperfection whirling against those seals, it just seems logical that the seals aren't going to last long.
Many years ago, I had a 40hp driveshaft in similar condition... actually a little worse. It so happened that a fellow that had a machine shop was at the marina and heard me telling the owner about it. He (the machine shop fellow) took the driveshaft, cut it down ever so slightly (completely around the shaft, filled it in with brass, then cut it down to specifications... worked fine, still is to my knowledge.
Something to think about.