When I did this conversion on my '78 CB750K, the old races had to be removed and new races installed in the steering head and on the steering stem. Didn't your kit come with instructions? Mine did, and it's a good thing, because there are extra spacers in the kit for different build/stack heights. Follow the instructions! It isn't difficult, but you can mess it up if you choose the wrong spacers.
Here's a link over to the SOHC4 forum thread where I photo-detailed the install. My thread was incorporated into a massive collected thread over there. My story begins on p. 6, about 1/2 way down:
http://forums.sohc4.net/index.php?topic=1516.125
Good luck with it.
Kirk