57 Chevy spring rate help

[Embryc10]

57 Chevy on a custom chassis with IRS. Having issues with ride quality and the car getting unhappy during gear changes. Roughly 700hp and it shakes the tires under throttle during gear change to the point it feels like it’s trying to jerk off the road. Current coil over springs are 550 front 500 rear. The car is extremely stiff. Any experience with a similar set-up? Are the springs too stiff? LT4/10 spd.

Thanks

[dhutton]

I have a 56 pickup with C5 IRS. It was a little squirrelly until I set the rear toe. I would also wonder if you have a rear bumpsteer issue. Have you experimented with shock settings?

What IRS is that?

Don