The fishplates ended up not looking bad. I am happy I went with them. If I had welded the frame in a better position and the bead looked better I would have just let it be but I didn't like the look and had to do something.
I wasn't worried about general warping but the weight of the cab possibly flexing the frame downward like how a 2x4 sags a bit if you only support one end. Probably not necessary but didn't hurt.
I tied the Z into the front. This is probably the best pic I got:
I did the SFBD first so I could just make one plate over the top.