Ok I'm not trying to stir the pot, but here is my two cents. Tried to explain it in the simplest way possible.
First, remember the old saying. "Every action has an equal and opposite reaction".
When the car brakes, nose dives, comes of a wheelie, etc... here is part of what happens.
First look at Figure "A" in the top left corner.
This is a section through the frame rail and rear attachment point of the K-member. Force #1 represents the weight of the car pushing down in the frame rail. Force #2 represents the force from the lower control arm holding the car up. In principal, this is what keeps the car "Up". You can see that these forces are not in line. What this means is that there will be a "twisting" of the frame rail.
Now look at Figure "B" at the bottom.
You can see the same Force #1 and Force #2. This results in the "twisting" that I was trying to simply explain in Figure A. The twisting is shown as Force #3.
So what counter acts the twisting of Force #3? It's safe to assume that most of Force #1 is also distributed through the front portion of the untouched K-frame. It is much larger/stronger then the rear square tube that was added.
As Force #3 twists up, it creates a Force #4 on the square tube. Force #4 and Force #1 (on the front of the K-Frame) are now "fighting" eachother to keep the car up. This is what normally happens with a stock K-frame.
But what you can envision, is that there is a huge mismatch of strength between the old front box section of the K-frame, and the new 1" square tube that has been added. The new square tube is clearly not as strong as the old boxed K-frame section.
To make things worse, the holes that are drilled for rack attachment bolts, is right through the worst part of the load path in the small square tube.
If Force #1 is hard enough, it will try to bend/break the new square tube because of the translated Force #4. This will most likely occur right at the rear hole that was drilled for fastener access to the rack attachment.
If it were mine, I would cut the square tubes off, replace with adequate size round tube, and weld tabs off the round tube to attach the rack. That way the tube is not compromised because of the rack attachment hole.
Just my opinion....but something to think about.
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Your going to kill yourself and/or someone else. 
