I just lowered the stock suspension on my KX500 8"
I didn't buy 450 bucks of Race Tech goodies.
I bought the normal rebuild components, seal head, bladder, bump stop, and oil.
I made my own lowering blocks for the shock from some automotive polyurethane suspension bushings.
I could have used all sorts of things even a old skateboard wheel
To do it right I should have bought new springs, I didn't. I cut mine and I should have changed the shim stack to stiffen them up and I didn't do that either
You should never cut your springs as you change the rate.
But for my build it does not really matter..
In your case you might have enough threads on your shock to compensate without cutting your spring.
Last, my guess is you would need to change the shim stack to make the dampening a bit stiffer but really, I don't know as I am not a suspension guy.
Again, for what I'm doing, my slip shod, back yard butchery will work just fine.
For what you are doing, just a little more work should get you tuned up.
Now.. if you still want to trade shocks I may have one.
I have two 83s and a 84 KX500..
Just let me know,,