You won't have a slipping-belt problem as long as the batteries are generally well-charged. Slipping belts are more likely to happen with more discharged batteries as more torque is required to put out more amps.I mounted my 12 volt alternator right above the stock 24 volt. I used longer belts and a replacement double pulley from a IH 966 tractor. The alternator seems to fit well in that place and so far neither pulley slips even though there is much less belt contact on the 24 volt pulley.
Nice set-up. GM alternator? Which one? Part numbers for longer belts?