You'll be 100% fine with 650# springs on a 2nd Gen. My 2nd Gen was on 620# coils with a bit of preload and handled great without the sway bar. My old 3rd Gen was 1,000lbs heavier than my 2nd Gen was, and it only had 590# coils with no preload and handled like garbage but we never died and trust me when I say I am not a conservative driver by any stretch.
I'm not sure how much of a difference it will make on a truck with a drop bracket. The sway bar is definitely a limiting factor but how far you're going to stuff the tires has a lot more to do with your shock's compressed length and the overall geometry of your suspension. In other words, it may or it may not. I would give it a shot to find out, just remember to take it easy at first... it might be sketchier than the rest of our setups since you have a higher center of gravity.