Dennis,
Butt welds on thin material are very tough to do. The best approach is to design the joint so it's a lap joint...the two pieces overlap. This is much easier to deal with and is often easier than all the exact fitting to get a butt joint.
Backing the butt weld joint with a piece of like material is also an option. Make a butt weld the least desirable joint on your shopping list!
Stuart
