What size spacers did you use on the front? I put a1982 GPZ750 front end on my sons KZ650 and while the fork seals don't leak oil, they slowly let air out and it's a pain in the butt, I want to add slightly heavier oil and put some preload spacers in to stop the dive in the front, it's too soft......
It will probably depend on the condition of your springs, and your weight. If I recall correctly I extended them by maybe 4cm. I took the caps off, put an old socket in both forks, and sat on the bike to check the sag. I tried a few different sizes until I found that a socket around 4cm long resulted in the correct sag (around 3cm or so of sag). I then cut down some steel pipes 4cm longer than the originals to make up new spacers. I've heard of people using PVC for spacers, it's not much more effort to do it properly in steel. It can also be a bit of a pain reassembling with the spacers in there, because you're fighting the preload.
Keep in mind this doesn't change the spring rate - it just changes the preload. I found the spring rate with preload is just fine. If you want to change the rate, cutting the springs and replacing some length with the same spacer length will stiffen up the front. You can only cut so much before you have issues with spring binding though. Or you can do it the 'right' way by just buying new springs.
