Can I use a 4x6 as a top plate for a stud wall?
I am confused as to the purpose of a 4X6 to support the roof overhang. Are you using rafters and collar ties? The rafter tails are usually what an overhang is created from. 

You should consider the conventional way of using  a double 2X4 top plate. The second or top 2X4 plate overlaps corners to lock adjacent the walls. A top plate of 4X6 would be very difficult to nail or screw to your studs without metal plates. I see no advantage to using an oversize plate. 

Maybe I'm missing the point of the question however.