The frame rails on the Impala are not a boxed design....instead they are an "L" bar. This bar will flex (unlike a boxed frame rail) a lot! The frame rails on my 81 El Camino (with an LT1) are the same, and I have broken 4 Hotchkis and 2 PST bars. On the last broken Hotchkis (they replaced all of their bars in a few days - terrific service from Hotchkis) bar I closely inspected the frame rails and say that they were showing some flex in that they were no longer straight, but rather kind of "wavey".
The LT1 is very "torquey" and will twist the frame.....do it enough and you'll snap the sway bars (any make!) at the bolt holes. It'll be interesting to see if the redesigned Hotchkis bar will hold up. I really think the only solution is to box the frame, and that is one hell of a job to do.