+1 that.. what he's saying is you can leave it in low and the computer and trans will shift at the highest shift point at WOT. So unless it's a manual valve body as stated its not very much difference.. But take it to your local track and run it both ways an figure it out.
I believe that's a shift kit. Manual valve body will allow you to be in any gear at anytime, save reverse. If you are at a stoplight sitting there, you can put in 3 if you are so inclined. I'm not sure if you can install that without removing the tranny. I had manual valve body on a TH350 I had put in my 84 Z28 years ago.