I got a STB to fab for the rear today. Before going any further I'd like to give a big
WTF!! to GM Parts Direct for charging me $10 shipping and handling and not bothering to put my STB in a BOX.
Now that that's out of the way . . .
Session 1 - 
After pulling the plugs that give access to the top of the shock mounts, remove this piece of carpetting from behind the seat. This will let you fold the side carpetting down low enough that it won't get in the way of your bar. You'll have to cut the carpetting later, but this will do for now. Put some weights on the carpet sides to hold it out of your way.

I used a nail to mark the area that needs to be removed to fit around the top of the strut mount, then used a hack saw and a cutting wheel on a dremel to cut the area out.

An extension will need to be welded on at each end of the STB.
Next step - find better drill bits.
Session 2 - 
I got my bits. I'm not sure but maybe all bits are good for metal and wood both, after all. BUT, I got a special set of split head bits that do not walk, how

is that? Finding them was an interesting experience in itself, they were in a semi-abandoned part of an old mom and pop store, the owner didn't even know he had them. I couldn't resist the old part of the store, it has the feeling of a forgotten ruin. I'm not sure but I think I got them at the price they cost 15 years ago. I love that store. I think it will soon be wiped out by a bypass.
Anyway, I drilled my holes in 3 stages because steel is frickin' tough. The instructions with my drill said to use "cutting oil" when working metal. I used motor oil from my last change. I don't know if it helped with the drilling, but the metal shavings stuck together in a more manageable pile.

Here's what the finished extensions look like.

The pilot holes. I did the curve part of the cut out with the drill, used a hacksaw on the sides, and then the bench grinder and a dremel to clean up the sides of the cutout.

Test fitting - it turns out the cut out I made in the STB is not too large after all, as it allows for the rubber caps to be installed. This not only looks pro, but the caps help hold the STB in place while test fitting.
Session 3 - 
I bolted the extensions down where they should be, and glued the bar into place with JB weld. I used a couple pieces of wood to wedge the STB down good on the extensions. When the JB weld dried I removed the assembly - one of the extensions broke off because of the tight fit. The JB Weld served as a guide to get the extension lined up right again on the bar. I superglued it back on.


This ain't Monster Garage, but I did get a FREEBIE!
FREEBIE!
FREEBIE!
(And I'm related to that Jesse James guy through Frank James, if family lore is true)
Anyway, free welding. WOOT!
The install was a PITA, because the shocks slowly retracted under the weight of the tops, and I did not figure out what the deal was until the end. But if you, dear reader, decide to try this mod, know that all you have to do is reach into the wheel well and push those puppies back up where they belong. You don't have to do like me and throw dead bodies in the trunk until it's sunk down onto the shocks again.
Here's the finished result -


It's clean looking if you cut the carpet in the right place.

For those worried about trunk space, there's plenty left, due to the way the brackets push the crossbar forward.
Driving Experience -
1st drive with the rear STB installed - IMO it's as good as the reinforced front STB. If you remember my comments following the addition of the reinforced front STB, I felt like the front handling was stronger than the back, and the back had become a bit squirrelly. I dialed my shocks up from 3 to 5, which was better but only a partial fix.
Now, the rear is most definitely quicker than the front again. The squirrelishness is completely gone. The back end is just fucking tight. Overall the car responds pretty quick, with the back and front starting to rotate instantly but the front going into a hard lean and plow. I say a "hard lean" to compare it to the marshmallow soft roll of the stock front suspension - it's not bad bad but it needs a big boost to catch up to the rear.
2nd drive - Dialing the Rancho's down took half the fun out of the suspension

I couldn't feel any squirrels in the back though - I turned the shocks up to 4, looking for a better balance.
For the 50-60 bucks it takes to do this mod, I don't think anyone will be disappointed in it.