Both pieces were 6061, the machine shop works with aluminum a lot (boats) and provided me with the 6061 as I had 7005 and he didn't want to mix the metals. He quized me on the metal type and actually only believed the metal type when I was able to show him the sticker on the frame. At that point he said he would be willing to weld it up as long as I used 6061 and then he provided me with the 6061 plate (1/4") to shape.

So, perhaps it will hold up? I know it is kind of late, luckily this is going to be a play bike on smooth pavement.