My were welded with just a regular welder. Went in fine although my mechanic said the "flooring area" was too thin to weld and wouldn't make a difference anyway. So he welded it to the heavy parts of the car where the stronger metal of th actual subframes are. Could be why I don't notice a difference in handling but I am doubting welding to the floor pans is an great place of strength.
Looks great and good long welds. He did not weld it with the wheels on the ramps per a recommendation of his long time Mustang owner/racer friends. The car actually sits better (like the gaps are perfect again, like new)and the doors don't sag in the least. and shut better than b4.
In terms of handling I feel no difference what-so-ever and bumps feel no different to me. Launch seems grippier better though. Other than that I notice absolutely no difference other than the piece of mind that my car aint gonna need a face-lift any time soon.
p.s. Obviously I am driving a vert but it has always felt solid to me. It came with a one piece cross bar in front underneath and another heavy looking more elaborate subframe partially welded just in the front right underneath the cheaper looking one.