If it cracked on you I would not even try welding because it will crack on you again.
The cracking is due to the fact that the ball head is too small and too weak for you camera, I would get a bigger (and stronger) one.

Try to get a replacement part for the ball head and use it for lighter gear in the future, and get a geared head like the 410 or 405 for your 4x5 from Manfrotto, depending on the weight of your 4x5.
The 410 is great for a Field and maybe a Sinar F, for a Sinar P(2) I would use the bigger 405.

Peter