First question would be are your hubs 36 spokes?
If so, is the front a disc or drum brake? If drum, you could use the 18" rear XS650 rim front and back (its what I did on my XS650 that has drum front brakes). The spoke hole angles on the rims need to be right for the size of hub you're running.
Spokes will be an issue possibly, but if the stock CB350 rims were both 18" then I'd bet stock spokes would fit. Otherwise, you call up Buchanan spokes and tell them the hubs and rims you're matching up and for $100 a wheel they'll make you custom stainless spokes and nipples.
Not cheap, but when you consider stock plated spokes are $30-$40 a set, the extra money is well spent for a super high quality stainless spoke that will last longer than you.