I had exactly the steering rack clamp problem on 2 of my chassis.
For one, I tried moving the clamp tops with a large adjustable spanner, backwards and forwards, slightly, any movement is fine, until the caps came off, then the rack comes off , then repeat for the two clamp bases.
For the other, I used a metal chisel to prise the tops off, and bought new clamps,
Yesterday, I actually re-drilled the 1/4" holes in the chassis, and put in two new bolts facing outwards. I cut a slot in the top of the bolts with a hacksaw, about 1/4" deep, put a split washer on the head so it does not easily rotate inside the box, and then use a 7/16" ring spanner and a small screw drive in the slot to stop the nut rotating.