At your weight, the 0.42 springs should be more than stiff enough.
Raising the fork oil level will greatly improve bottoming resistance and stiffness as the fork nears bottoming. To measure, take the springs out and bottom the forks. See how high the oil level is from the top of the tube. I think you can raise it as high as 4". If your level is way to low (6" or more from the top with forks compressed), raise it to 4.5" or 5" for starters. A .5" change will make a noticeable difference in bottoming resistance and the last 2" or so of fork travel.
However, the classic problem with the old KDX forks is too much high speed damping. Basically, the fork does not move rapidly enough on rocks and roots even with the wimpy stock springs, but its still way too soft on jumps. It more or less does not move enough on rocks and roots, so instead of bottoming, you might be locking up. If your oil level is already high, you might have a damping problem.
Race Tech cartridge fork emulators will work wonders on your '89 KDX fork, and allow the fork to respond and use all its travel on rocks and roots. If you ride lots of rocks and roots, the emulators are a huge improvement.