I found a company in Japan that makes 33mm dual disc fork lowers specifically for the CB350F/CB400F. They also sell the corresponding dual disc wheel axel bolt and what not!
Do you want to share?
No...because then as soon as I share them everyone will buy them before I can find my expiration date on my card. lol. I'm just kidding currently the company is out of stock. One of my friends who speaks fluent Japanesse will be talking with the company to see when they will be available. For all we know they aren't making them anymore. I hope that is not the case! Will Keep everyone posted!
So here are the items they sell that you need for the kit
Replacement Axel bolt for dual disc mounting (MSRP $46.87)
Lower Forks. Obviously (MSRP $330.00)
Speedo Cable with bend so it doesn't interfere with additional break (MSRP 30.00)
Slimmer speedo gauge retainer (MSRP $80.00)
Right side brake pipe (MSRP $12.00)
Longer Bolts for dual disc (MSRP $20.00)
Those are the parts they have (if in stock). The obvious other parts you need are an additional brake line and dual banjo bolt. Also the super obvious (additional caliper assembly, arm, & rotor.
You can switch out the master cylinder for the GL1000 just like the fellows do on the cb750's but the company also makes a bore kit for the original master cylinder (MSRP $94.00)
There I will let you know if they get back in stock.
-James