SS Spring Advice

Just want to ask for any advice on set up on SS Springs.

I've got a '73 Duster that recently ran 13.10 @ 106mph but with a lazy 1.9-2.0 60ft times so it was spinning off the line.

I recently put SS Springs (3600lbs), Competition Engineering Rear Shocks set at 50/50, fronts are 90/10 and it's got 4.10 Sure Grip and it did this time with brand new M/T ET Radials (275/60/15).

Anyone got any tips to fix this issue? Should I try different shocks, spring clamps, slapper bars or Caltracs?

Thanks in advance for your assistance..