The 3.5 springs may do a good job, but the 4.5 are a bit stiffer and are a straight fit. Given the weight and performance of your engine, I would go straight to 4.5 springs and sway bar. This is what I ended up doing, with an engine lighter than yours. The main difference among the springs is the wire diameter. So before procuring 4.5 springs you need to make sure the wire diameter is bigger than the 3.5 ones you have. Someone with a workshop manual handy may tell you what the diameter of the 4.5 spring wire is.
The front leafs connect the front axle to the front cross member. You should fit the shortest you can find for more caster. The car will track better on the highway. I have W109 6.3 ones fitted on my car, I think some W110 ones are also shorter. Again, exact lengths can be found in the workshop manual (that I don't have with me now).
Coxracing Group does the polyurethane bushings for our cars. The guy is specialized in motorbike aftermarket parts but if you call him he will send you the bushings. He will ask you what diameter sway bar you are using: http://www.coxracingroup.com/Contact-us
The 4.5 brakes will be a straight fit, unless you have a very early front axle. You will need the discs and the calipers. Replace all flexible brake hoses while you are in there.
Before doing anything with the drive shaft I would make sure all mounts are in good nick (including the cylinder shaped one at the center of the rear axle). Trans output flange and rear axle input flange need to rotate in parallel plans. Until you adjust engine and rear axle positions for this to happen, you will have vibrations.
As Tony said, check the coupling between the steering column and the steering box. You may also check if you have play in the kingpins. Looks like you are in for a busy winter as most of what you need to do are better done at the same time, not to repeat same operation several times.
Some info on my car here: '62 220SE Coupe 5.0 Presentation