I would have thought that a 265 is a bit too wide on a 9" rim, and 235 is a bit narrow.
Also, try a heavier rear bar. Run higher air pressure in the front tyres.
Are the coilovers adjustable for ride height? If so, have you had the car corner weighted?
On a different tack, upgrade the brakes to use braided lines, fit some good pads and slotted rotors (if you haven't already).