If a problem does arise it will be at the point of maximum cyclic stress which will be somewhere in the vicinity of where the forward crankshaft bearing journal meets the camshaft gear. The diameter of the crankshaft extension is reduced at that point which is always an invitation for fatigue issues. From that point outward the stress will reduce as the moment arm reduces to the end of the shaft. The diameter of the adaptor won't have any effect on the bending stress in that location. Hopefull the Volvo is so over-built it will never be an issue. If it were my engine I would probably put a dial gauge on the edge of the new sheave and apply a load by squeezing the belts together and see how much deflection there is. That wouldn't give any usable numeric data but could indicate if there is a problem

Further to the question Tom originally raised about the need for the huge alternator on a day sailor and the need for power. All the winches and other loads still won't need so much power so I would think the owner also puts in a large battery bank. That way they can sail using the bank and when it's time to charge they can pump lots of amps into the batteries to shorten the engine time. That' the philosopy on our boat. In summer cruising mode we need to run the engine about 2 hours every 3 days to keep our house bank in the 50-85% charge range. Beats the heck out of the boat we charted in the BVI last month where we were told to run the engine 2 hours in the morning and 2 hours in the evening! It turned out to not need quite so much but it was still a PITA.