Isn't it camber that would be affected by different arcs rather than caster?

John, you should describe better what your car is doing to help us diagnose. Is it pumping up and down at speed? Or does it have difficulties tracking staight on the highway? ie do you feel insecure passing a truck when you have a wall on the other side while being in a curve, because you feel you can't really control precisely the trajectory of the car?

I'm no specialist, but this reading helped me understand better how toe, camber and caster work, interelate and act on the car's handling: http://www.ozebiz.com.au/racetech/theory/align.html

I second what Jay said: first put all new bushings in the back as a loose rear end greatly affects handling. In my case worn bushings were having me very nervous in curves as I had the feeling the car would go oversteering without being able to control it. I doubt this would have happened in the reality but it was the feeling I was experiencing. If your car has a tendency to go left and right under heavy acceleration this may also be due to worn rear bushings as toe of the rear wheels changes and sends your back end left and right.

Tracking problems is more related to the front end, and as docsaab said, can be corrected by more caster and by making sure your toe is in, not out.

I've also learned that spring rating and shock stifness are closely interelated. Putting too stiff shocks will put that relation out of whack and end up with bad results. So, if you go for higher rated springs, make sure you mate them with the correct shocks, that may end up being softer. This may seem strange, but I have some examples of this if you need them.

Finally you say you added 200 lbs with the V12. You also told me this was 150lbs heavier engine and 50lb heavier transmission. I wouldn't worry too much about the transmission as it is quite in the center of the car. Your engine is 150 lbs heavier than the alloy V8, but only 60 lbs heavier than the iron V8 found in the 450SL/Cs. If you move your 40lbs battery in the trunck (if not done already) you just have 20lbs more in the front, that may be partly taken care of by your alloy radiator, power steering pump and A/C compressor bracket. And the weight of the battery in the trunk should really put the weight distribution of the car back to where it was before the swap, if not improve it. All that to say your 450SL/C front springs may be up to the job. I would look into chassis adjustment first before replacing them for stiffer ones.