Here's how I did a few years ago when my frame cracked at the steering box.
After reading about how prone to cracks these trucks are, I removed my steering box to inspect the frame just in case.
And yes mine was cracked as well
There was a crack in the frame from the front most hole and upwards to the edge of the frame.
I grinded it some and then welded it up. I completely filled the "notch" on the top of the frame with weld.
I can't understand why they would place a notch there, I think it's pretty obvious that it's a weak spot
Here's the steering box plate after I welded some cracks in it.
I think the biggest problem is the lack of support for the upper part of the frame in this area,
So I welded an angle iron between the frame and the crossmember like this