Well the set I installed on my '71 Dart definitely required a lot of fitting and grinding to get satisfactory gaps for welding. It took longer to do the fitting and grinding than it took for the surface prep, including removal of the factory undercoating. Heck the undercoated areas were easier to prep than the areas without undercoating because of the surface rust removal needed on the spots without undercoating. The factory undercoating was a piece of cake to remove, just a putty knife and some elbow grease. You can see the nice sharp edges left by the putty knife, plus the extensive coverage of factory undercoating on my '71 from the Windsor plant. I did a quick wire wheel following the putty knife for surface prep, but that was pretty quick.
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The tolerances on these cars aren't all the same, so I'm sure they install easier on some cars than others, just like everything else. And compared to a tubular set of frame connectors they require a TON more welding, at least if you fully seam weld them. From a strength perspective that's probably not necessary because you're just welding to sheet metal, a skip or stitch weld with a decent amount of coverage would probably max out the stiffening effect gained. Somewhere between spot welded and fully seam welded you'll hit a point of no additional gain.
I wouldn't say "can't", but you shouldn't anyway. Like any long stretches of welding on sheet metal you should break it up and move around to minimize warping. I was doing ~4-6" stretches on mine depending on how flat or contoured the floor was and moving back and forth between the left, right, front and back.