I've modified my exhaust using pieces from Summit. I soon plan to put together an entire dual setup doing this. I use $20 mandrel J-bends and U-bends to piece it together. I have a couple ideas for routing, but I'll have to see what works once I put it together. I just cut the J & U-bends into the necessary pieces to accomplish the overall bends I need.
I've found that exhaust pipe welds best with a little more heat than you'd normally use for the given wall thickness. I think it's because it's aluminized.