Is there an amp. difference in alt. between a 1972 240 and a 1975 280. If yes, then what. Would I need to follow the conversion guides out there like they have for the 280zx models?

No conversion needed.The external voltage reg units are the same except because the 280 had more electric requirments it has a higher amp charge.They are a excellent swap.The internal 280zx unit had internal volt reg and did require the conversion you mentioned. They all perform the same task,they just charge the battery.