spring suggestions

Another newbie here, I'm new to circle track, this year I did a few races with what was in the car when I got it, and I know it needs something, i'm just not sure what it is. The car is older, it has a 68-72 chevelle stub and a 3-link in the rear. I dont have a spring rater, so instead of going by what is engraved on the spring I was going to just buy new ones, so I know they are what they say they are. the fronts are supposedly a 650 and a 750 with 200 on both sides on the rear. I'm just not sure what a good starting point would be, our track is dry slick about 90% of the time, so I think I want to go softer. The track is 3/8 mile with banking, the shocks I have are S7Z3050 on both fronts. On the rear, the only numbers I can find are 9500, must be a 50/50 shock? The springs are on top of the housing. Would I be better off putting them on sliders and moving them or just leave them on top? From what I see in the pictures, the left rear barely lifts at all. When I come into the corner I have to run mostly rear bias on the brakes and sometimes momentarily lock them up to get the car to rotate into the corner without pushing. If im in the middle or up high on the track it wants to push going in and in the middle of the turn and is very loose coming out of the corner. The only time it really worked well is when I would go way low, so either there was better moisture down there, or thats how much force i had to put on the suspension to make it work right. I'm leaning towards the suspension being too stiff. Any suggestions would be very helpful if there is any other specs you want to know ill do my best to get them.