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First, set the camber on all four corners to something like 2 degree. basically, when you look at the car (at eye level) from behind, the wheels should look like this / \ Obviously, not that much, but in that general direction.
Front toe can be -1, 0 or + 1. as Long as your in the ballpark, I don't think this is causing your problem.
I think you don't have enough rear toe in. I would start by trying 2 to 3 degrees of rear toe.
You don't need spendy setup gauges for this - just use the plastic AE tool (Set the car vertically on the bumper to check toe with this gauge).
Are the suspension arms level, or is your ride height higher than that? If its higher by much, you might need to find a different (smoother) surface.
Yes, body choice as well as a rear wing will make a noticable difference. In your case, the wing is the biggest factor.
Also, check to see that all four wheels are getting power. Things like damaged drive cups, missing pins in hex adapters or damaged outdrives can cause this. Be sure to not overtighten the wheel nuts. This could cause binding and cause bad handling.
good luck