I have a similar situation. My '52 Ford F1 pickup has a '79 Monte Carlo front clip, installed by a previous owner. I have been doing a major overhaul of the truck. The problem I have with the steering was that it tends to wander, as well as too much slack in the steering. I installed a new PS pump, new steering box, all new A arm bushings, ball joints, and offset upper A arm pins to gain more camber.
The truck drives much better now, but the steering is still what I would consider "darty" because I can only get the caster up to -3 deg without sending the camber negative. The negative caster is what causes it to wander. I'm currently evaluating options on how to get more positive caster (which is how I found this thread). Moog makes upper A arm bushings with eccentric holes that could add caster, but I haven't found one that fits the G Body control arms.
Several vendors makes adjustable tubular A arms, but they're expensive.
Another option that I'm considering is taking it to the frame shop and having them bend the crossmember to give me enough positive camber that I can use shims to get more positive caster.
By the way, it's pretty easy to eliminate your power steering pump as the culprit. I got mine up to about 50MPH going downhill, slipped it in neutral, and turned OFF the ignition (which renders you NO power steering assist). I found that my truck still wandered even with the engine OFF, which told me that it was somewhere in the geometry and not in the hydraulics.
-DV
