you are doing a great job, but instead of butt welding you should get a lip former tool from eastwood, that way your metal will fit into a groove making the seams alot nicer to weld, my 2 cents worth,
saw another member do this tought his was great so I tried it myself, wasn't to bad,let me know what you think.
http://s626.photobucket.com/albums/tt342/padgett_1970/?action=view¤t=3bc18888.pbr