Light and medium duty truck frames are made of mild steel, they are not heat treated, as this material does not respond to heat treatment. I don't recall what filler material was used at the frame plant, but I am pretty sure it wasn't over 70 KSI. They were welded by MIG with CO2 shielding, no pre or post weld heat treat. I specifically remember the specified carbon content being 0.05% MAX on at least some of the frames. The heat treated frames found on heavy trucks were made from SAE 1027 modified. There are legal ways to lengthen these frames, hoops, hurdles, testing and certification, not something to be taken lightly, even if experienced in other areas of welding.