I would'nt be scared to let a muffler shop weld it. They usually have their machines already set up for that thickness of material. They know what it takes to seal it up right. It's a very easy weld to make, especially if the machine is set up already.
What I would worry about is how much warpage the mateing flange would get. Thats a lot of heat on a small section of plate.
Make sure you know the orientation "exactally how they fit" when you take it to someone. (if it matters) You dont want it welded on backwards.

J-Bird

'95.5 T, SR5, Bone stock w/32x11.50's
'06 Sonata LX