Hey Bill,
On my car (while it was running there for a few months) I clearly didn't need a rear sway bar as the car would oversteer like a mofo... needing a rear bar will somewhat depend on a lot of parameters, but a quick way to know is whether it oversteers or understeers in a turn. If you push, you
might need one, but if it oversteers you definitely don't want to stiffen the rear up any more.
Also depends on how you drive. I needed to get understeer through one of the turns at Willow, so I went in hard and just before the rear wanted to come around near the apex I put in some throttle and forced the car to push through it. Big torque V8s are nice because throttle steer is always on tap. Takes some balls to do though - most people are afraid to power down when a car is "upset", let alone create slippage as it seems like you are just adding fuel to the fire... and on one lap I went through that turn then looped it on the next one
With all the changes I have now my story may be completely different... but with the low polar moment these cars have and the rear weight distribution the V8s have, you probably won't need one, unless you do some serious tracking of the car and a lot of suspension tuning in conjunction with adding the bar.