Look for some recommendations.

Stock 914-6 with stock T bars, front 22mm H&H Swaybar - adjustable, and rear spring rate of ?????? I was thinking 170.

Shocks are all Bilstien Sports, springs are 2.25" size so I can pick most any rate I want including variables, just not sure what to run on a street drive 914-6.

What kind of rear suspension action are you looking for?
Sport or soft? What are the road conditions that you are on the most?
I have a 4cyl water cooled conversion@ 2200 lbs and use 225lb springs and like them. Good for most roads, but they do sacrifice compliance on the rough stuff. I would put these in the sport category, but very tolerable for long distance drives.
BTW my tires are 205/55/16's.

Why not progressive?

Sport, I don't drive anything that is stock

I've driven a narrow body 3.2 conversion with 180# springs that felt harsh and a flared 3.0 with 225# springs that felt fine. So I have no idea but I think each car may be a little different and you might just have to give it your best guess. I have 165# in my car now but still not on the road. I don't think I can go any lighter with my 3.2 and still hold up the car.

http://www.hypercoils.com/spring-calculator

Sway bars change the rate calculation....there is another site that calculates that as well but I can.t find it at the moment.

I would add that good dampers play as much role in ride comfort as the spring rate.

I run 200# springs with my 3.2 and I think its a good balance. I think its all a personal preference. Also I've heard anything over 200# starts to put additional stress on the body. If your not track racing it, its probably not a big deal though.

Also how tall of tires do you have? The less sidewall you have the more spring your going to want.

Mine are 200# in the rear with my 3.2...