So I finally got around to fixing the frame. Took your advice and decided to keep what I had. Welded up the cracks, added a piece to fill the void. I was a bit concerned about the overall strength of this section of the frame because it sees a lot of load from the shackle mounts - so I boxed it in. This is one of my first major welding projects and although a bit ugly I'm pretty happy with the results. Comments and criticism is welcomed!