I agree with Michael, and I'll offer a couple more details. If your current 5/8" engine-side clutch half has a captive cotter pin, there's a likelihood that you'd have to replace it at some point with the later type which had two set-screws at right angles instead of the cotter. The cotter type has reliability problems, the underlying cause of which is uncertain at this stage. However it is known that with the cotter, the key that drives the clutch from the crankshaft gradually slides along the keyway until it fouls on the thrust bearing in the middle of the clutch. The set-screw type of clutch half has one of its set-screws directly over the key, ensuring it does not move.
So, it is possible that you will eventually need to replace the engine-side clutch half anyway, and doing it now will not cost any more than doing it later. I'm told the clutch half costs about $80.
