I saw some of those posts, one from a former owner here. I totally disagree with him; a missing link is a must for SFD trucks...and not something made from angle-iron, rebar, or conduit; not rigid enough. Non-SFD trucks probably see less benefit because the subframe is no less rigid than the chassis it's directly bolted to.
This is what our subframe looks like naked:
Top
Bottom
As you can see, there's a single crossmember that only provides rigidity towards the front of the subframe. When it's bolted to the chassis without SFD, the chassis adds rigidity to the rear. When you move it away from the chassis via SFD, the rear of the subframe can flex. The missing link boxes in the subframe somewhat, reducing flex.
Those damage pics are from my buddy's truck, @Jax99. His spacers are identical to mine...in fact, I had them made and I did that install several years ago, shortly after installing my own. The difference is that I've had a missing link and skid plate the whole time. He's since had the subframe replaced and the chassis repaired, with the blocks welded on in that spot and gusseted to the subframe, plus the truck now has my old missing link and skid plates.
But wait, there's more...he recently struck a curb on the same side he had subframe damage. This is the difference of damage transfer when a missing link exists and the spacer is reinforced:
For yours, I think you need similar gusseting. I'd say weld them to the top of the subframe if needed, but you may also need to consider a way to box in that 6"x2" tube.