With William Wallace I was trying to find out when came to Australia, I have looked all over the place with no luck, I see on the web other people have not found the answer.
Maybe you might get lucky with William's death cert. and it might note how long he had been here for as well, keeping in mind on how much the informant knew and i would work a year or 2 either side of that. Might help narrow down arrivals.
His death cert. should also note all of his kids, alive and deceased at the time of his death so that will help clear up any confusion with his off spring.
Just for your interest if you dont already have it