First things first
First thing here is don't start over analyzing the problem. The FIRST thing to consider, is do you have LSD? A car making the power you are making (419 @ crank) with no LSD, and good hooking tires, will cause your car to go left when on the power. If and when you rule out the LSD question, then look onward.
And don't cut yourself short, you are making more power than most on this board with the 356 RWHP you are sending the the rear wheels. Curious as to the mods you have on your little beast!!!!!!