I did this many years ago for a similar instance involving SGC and an after hours call out allowance.
In my instance, the ATO ruled that the allowance did not attract SGC.
I guess from the ATO perspective, any income (eg, allowances such as site, dirt, etc) directly involved in the ordinary time earnings will attract the SGC, but any income outside for the hours will not - such is life.
If your CA expressly indicates that your OTE for that that job are 60 hr /week, then that would be a good starting point for the private ruling Good luck.
PS: from memory shift allowance did attract the SGC
