I had a similar problem with my E-body Cuda years back. I adjusted the ride height between front and rear until it road steady at 110mph. It also has a "Go Wing" on the trunk with was helpful. You can feel the car sit down at speed.
One night, I found an open road @ 3 am. So I went for it. The car topped out at 155mph and road like a brick sits. Try dropping the front a 1/2 inch and test.
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