Mounting half fenders

I would weld a metal triangle where the horizontal piece and the vertical piece meet. Weld it on the front side to help support that stressed point. I can't tell how much room you have but you'll want to keep the brake chamber accessible.

 

That angle iron cross brace looks like a good idea but should be higher so it’s inline with bracket. The way it is now it puts stress on the area where it hangs below bracket . Least that’s what it seems judging by pictures.

 

What brand are the fenders PAV?

 

I don’t know if anything is going to work. Your frame rails flex and move independently of each other. You could try welding triangle braces on, but I think they will crack the flat stock either up higher or crack the angle iron.

 

I would add two triangular gussets on either side of the cracked vertical to stop the flex from the weight of the fender hanging there and thicken the cracked vertical piece.

 

Heavier steel. Fenders are heavy and vibrate like crazy. Go way overboard. My .02