I'm going to assume 3400# with maybe 54% on the nose. That's about 1550 on the rear.

What shock/spring combination is on this? If it's a typical 17" extended length shock, like a 3850 AFCO, or S5206 or S5006 Strange with a 12" spring, a 200# spring has to be holding that shock at full or almost full extended length. It's can't separate without topping the shock. Plus it has to about as stiff as a solid suspension.

So, the shock installed length needs to be correct, about 13 3/4" or so. I would try a 150 if my assumptions are correct. This spring will need to be compressed a bit to hold the shock at the correct length @ ride height. Then a good shock can be used to control the separation at the hit. With that horsepower level, it won't need a lot of separation. You don't want to crush the tire.

90/10s are probably not the best choice for this car. A good adjustable shock, so you can control separation, with a bit smaller than stock torsion bar is where I would start.

Again, this is all based on assumption of what's under that car.