As Luis mentioned, the welder will start the pool on the thicker piece and bring it down onto the thinner piece. The strength of the weld will be the same regardless of the thickness of the top piece. The thicker top piece though will be less apt to tear and the welder I'm sure will do a good job securing the 2 pieces. It would be great to have him (or her) weld all the way around but I can tell you that from my experience it is quite difficult to get around the front side of the tube but again, I was welding to the base of the tube and not 2/3 of the way up. Either way though I'm sure you will be fine. Please take lots of shots, this is a topic that will come up again and again unfortunately.