Hi Jen,
I too had my neck dissection about 8 weeks post-rad. It was a selective dissection, so I'm not sure if it's comparable to what you're looking at. Mine took about 3 hours,and they froze and stained the samples, and looked at them right there in the OR. I was allowed to go home that night. I had a draintube for 4 days, and returned to work (part time) 6 days later.
As many have said, the surgery was a breeze when you have radiation treatment as a point of reference. There was some loss of range of motion, and strength in the shoulder, but now a year later it's just a memory.

You can do this.
Best wishes for a quick recovery,

Chuck