This may give an idea of the way the tube work goes through the upper flat of the frame. Because of the way the tubes curve the lower frame flat does not turn out looking as clean. Had to trim the frame back and make fill plates. Also had to notch the frame behind the rear upper mount to make room for the upper a arm.
The frame has sorta been turned into Swiss cheese. I thought I planned my cuts better than this. Of course cutting a big hole in it to clear the rack and pinion doesn't help. I'm planning to box the frame from front to back. Based on the previous comments and the way the frame looks I'm going to add a 90 degree bent piece of tube from the ends of the lower control arm mounts up to the frame.
The other thought is that the only time the sub frame should be holding the weight of the truck is when the truck is on a jack. The coil overs or shockwaves (haven't made up my mind yet) will be taking the weight then transferring it to a bracket on the frame.