As someone that worked as a certified code welder for almost ten years using GMAW (mig) and GTAW (TIG) on metals from less than .035" up to 1/2" I have to say that you "could use mig" if you are not using a "hobby welding machine". You will need thin diameter wire that is compatible\complimentary to the composition of your base metal. You will need some pretty fine control on your settings, and your choice of gas will affect the properties of your arc - which in turn affect penetration and finished appearance of the bead. Way more than I would want to deal with when a better option is available - TIG..... If you are talking modifying a welded frame, it may be easier to replace the top and down tube entirely. You WILL need a fixture to hold it all in place as alignment is critical - yeah I know I'm stating the obvious....
And then there is the relationship between your head tube angle, your rake\trail of the fork, and how the change in weight distribution figures in....
A lot to consider for a DIY project that could easily end up a pile of scrap metal.... Are you in school for welding, working in a fab shop, or just like burning wire? Why not get a tube set and build one up from scratch?
Last edited by hockeyref; 07-10-13 at 02:37 PM.