Changing to rack and pinion is far harder than it may seam. I have done this once. Don't do it unless you have a mind for engineering problems, and keen with both a lathe and a welder.
I surgest you get the steering box reconditioned, or adjusted at least. Replace the idler arm, (cheap enough). And replace some of the ball joints in the steering if needed.
Also get a wheel alignment done, with lots of caster, more than stock. Steering box is ok.