No E-body here, but...
I had always heard that weld-in connectors were stiffer then the bolt-ins. Over a period of time, the bolts loosen, holes elongate, and then you have some frame twist again. Not just on E-bodies or Converts, but all cars. I put a set on 1 Duster and 1 Demon and was night and day difference. I bolted and welded them in. The Duster was manufactured parts and the Demon was bought parts/kit. Bought it thru a company- Super Shops in Kansas City about 20 years ago.
There is another set on the market now that is cut to follow the contours of your floor. Of course this is weld-in. Looks like it would support and strengthen the car more then just welding the ends of another kit. If I had to do it again, I'd buy this kit and weld it in solid! after it was welded I'd paint it same color as the rest of the body. On some I've seen, It looks like factory when done right.
Some people don't like the idea of welding it in because it "changes the value" of their numbers matching, low mileage, XXX car. But you really don't hear much about people trying to reverse many mods they put into their car that makes it better.
The bolt in kits can be welded in also. Just depends on what your looking to do to it and if you are looking to sell it in the future or not.
You'll be pleasantly surprise on how it handles and feels after the mod!