Sandpaper and elbow grease will tell you what's the real condition. Don't be fooled by the appearance.
My CB350 forks were also rusted under the triple tree. Check the picture: They are both tubes that were rusted very similar; it's obvious which one has been sanded -a 5 minute job-. I will reuse them with complete peace of mind, either painting the hidden part previously or thoroughfully greasing them to prevent from future rust. Sand them and check the severity of the rust. If it's just surface then no problem with strenght. Furthermore, the strenght is only needed at the lowest part, where the tension is exerted. There is no side tension between the clamps.
Raul