My tires are soft sidewall. Some feel that a car 2200lbs and up needs stiff sidewall tires. I disagree. Every car is different.
A buddy of mine has a 72 Chevelle, 2800lbs, 565 ReherMorrison, has run 9.08 @ 148mph up here in Colorado, around 7000ft that day. He's tried stiff sidewalls and it just spins, so he runs soft........6.5lbs most of the time.
Are your tires old? I mean age, not number of runs.
If it was me, I'd start with 5 1/2-5 3/4lbs next time out and see what it does. If it shakes or handles weird, go up. JMO
Johnny