Mark, if you want modern day suspension on the 66, get 0.5" taller lower ball joints. Trust me. My 64 would jitter over bumps, bridge abutments, etc, really unsettle the car. After the taller lower balljoints all the bumpsteer is gone, I'll drive this car anywhere. It will lower the front end .5".
Keep the KYB shocks, I have them on my car and am really surprised at how well they work for the $. The car rides better than anything else I own. It's butter smooth but will take a curve like you wouldn't believe.
I've LS swapped my 64 4 door and am running Moog 6200 stock springs on it, it's the best riding vehicle of everything I own.
Looked up 283 weight, web is saying 545-615lbs. My LQ4 6.0 LS motor is 527lbs. You might want a touch heavier spring than the 6200 (290lb) depending on what kind of ride you're looking for. I'm driving this car like a crackhead stole it and have yet to bottom out. My daily is an 07 Yukon 4wd, the car rides better than it.
CPP 2" lowering spindles, SPC fully adjustable uppers, ebay lowers, proform 0.9" tall upper, 0.5" tall lower, moog 6200 springs, KYB gas-a-just shocks, Hellwig 1 5/16" front sway bar. It's a winning combo, trust me. You can click on the link in my sig, go to the last page or 2nd to last and see the ride height. I've got 14k miles on this build, 12k with stock suspension, 2400 with this front end setup. Only regret is not doing it sooner.
Use this Moog page to help find the coils you need. Don't need aftermarket coils, these work great. I'll be autocrossing end of this month, stay tuned for pics/vids. You can see how well this combo works.