-6 is better than zero. Yes, handling may suffer pretty bad in the curves as your tires will rotate wrong and the contact patch will get bad and you may lose traction, but for going straight down the highway I would rather have some caster, even the wrong way, than nothing at all. At least it should help the steering recenter itself. Hopefully you had the suspension setup for the correct caster when you welded it and you just reassembled it with the wrong caster? Otherwise I would be worried about stuff colliding into each other, moving it 11 - 14 degrees is a lot! I hope you can simply adjust the arms and everything is better! Good luck.