Hi Guys .. This is my first Post . Solidworks Version is 2013

Basically Need Help Mating one circular Tube onto Another ..both have Different Radius

I have a Main Circular Tube with a Certain Radis. Onto this tube another Circular Tube( Different Radius so its hard to imagine how it would Sit under Coincident Mate)  has to be welded or Mated ( Mated I believe is Easier ) . Then i have a Part that Cover a Slit on the mail Tube .. I dont know how i can Mate this Assembly .

Also If you Look Carefully You can See The Hole in the Bottom Picture over which these Parts are Coing to be Mated ... How is this part going to sit over the circular hole ( luckily the part is circular at the bottom as well. ( Both Mating Part and the Mating Surface are Circular and have Different Radius on them ..)