727 jumping shift detents

You would have to carefully remove the valve body to see the top, as in the photo. I never heard of any mods to the OEM detents or OEM shift mechanisms. But you would need to find the correct spring in the valve body, if that's part of the problem.

You should be concerned about the trans upshifting or downshifting a gear when you don't want it to. Aftermarket shifters work with the OEM valve body for precise control. Although some people do it and some are pretty good at it, it's still risky. An aftermarket shifter is not only safer but will likely be more consistent as far as your shift points are if you're bracket racing. One less distraction you so you can focus on cutting a light, hitting your shift points and judging the finish line.