Are trac bars really necessary on my 79 Camaro with a mildly modified 350 auto, open diff and 2.41 rear gears?

 

With your combination I doubt you are going to run into leaf spring wind up problems requiring the use of slapper bars. However, if the bars are already installed, it won't cause any problems.

 

If you're in the habit of wanting to leadfoot the gas pedal off the line, your leaf springs will be wrapping up some. Maybe not enough to cause serious traction loss, but slapper bars can help to prevent spring fatigue/breakage. If you do add some bars, don't get the cheesy clamp on style, use the good ones that replace the lower spring plate.