Man that is the million dollar question
-Get rid of the small quad shocks.
-put new rubber bushings in your top control arms.
-I am up to 350 in the front and 225 in the rear for the street, but the biggest difference is in your tire selection side wall stiffness and height that dictates what is tolerable for street use and why no one can give a straight answer. You want some preload on the the springs to keep them in location under compression over bumps AND in place when the car hops and is unloaded, this is when expensive parts come out of their seats and major misalignment happens. So you crank up the spring and the car lifts up and now you have a rally cross car. The higher spring rate turns the car into a brick but looks cool and low and will go around a corner fast but your teeth and kidneys hurt, but it is a race car...So for the street I pick a tire with a 55 side wall height and around a 98 in stiffness and I get ragged on, yes there is more side wall flex around town with a HP all season tire but I can drive in the rain and the pic of my car passing a GT40 in the rain is on here somewhere. I am on Toyo R1R's in the summer now and the difference is night and day with the same suspension settings, harsh, twitchy, fast response, much higher grip. I would have to change much to make it livable on the street but that is not there purpose. So how cool do you want to look? What is important to you? Everything ids a compromise.
People make up cars I see and drive that are terrible and need to go back a few steps and that is why they are for sale, just too cool for the street and not competitive on the track because they can not be driven at 10/10's.
A longer spring is going to have to be softer to stay at the same ride height or you run out of adjustment, bounce comes from the spring pushing the whole car up under compression, the shocks job is to compress easy and release slowly to stop bounce so the shock on compression could be too stiff.
MM is great to talk to but they can not provide the magic solution, there is none, we are working with a Ford Fairmont chassis. This is why new cars have multiple settings and competing against a Lexus on the track is ridiculous. So the SN95's that are maxtrack around here are 600 front and 400 rear.... might as weld the suspension solid IMO but they tell me "oh you can feel the car move".
Pick your poison. I could not stand the harshness of solid engine mounts on the street and went back to stock with a Kevlar torque strap... same difference, no movement. I love the handling and response of a 200 race tire but I do not want to drive to the track with them on, I am worn out by the time I get there, they are made for feedback and they do.
So I think start with the tire and work backwards, they are the most important part of the suspension.
