they stopped making Cb500 fours in 1973. You mean CB550?
they have 35mm forks just like Cb450, Cb750, xs650, etc....
CB750 are a little longer. Cb750F are 35mm too but even longer.
CB450 are same length i believe.
what are you trying to accomplish? just need some forks that aren't rusted or bent? or looking for something longer?

Shaun