I did the same thing years ago. Late June as I recall. Permits NA for Boston Basin, but we were able to camp on Sahale arm and traverse over for the Sharkfin climb. I don't remember there being much difficulty; some snowfields to traverse, not too exposed, but no place to screw up either. Depending on current snow conditions there could be lots of talus walking. It's a fairly long way across Boston Basin to Sharkfin, but it doesn't seem we lost that much elevation once we got off Sahale arm.