Putting an crank with an ISO interface on a JIS spindle could cause the crank to bottom out on the taper. If that happens, you won't be able to get the crank tight enough on the spindle to keep it from wobbling and ruining the crank. It's not likely to happen, but the only way to know for sure is to try it. As an alternative, consider punting the Maxy on ebay and purchasing a crank like the FSA gimondi.

It could be worse though. You could be building a French bike.