I for one STRONGLY DISLIKE whitewall tires on mag (or semi-mag type) wheels, or on anything but actual SPOKED wheels; I think the Honda Comstars on your CM would look ass-ugly as all hell with whitewall tires. I'm really not aware of any real cafe racer that wore whitewall tires. Sometimes, the best ideas are the ones you ignore; whitewalls just don't cut it on a genuine cafe racer...
You might somehow be confusing "retro" with "cafe", or apples with orangutans, or a gizmo with a doohickey. In a different thread, you mentioned that you wanted to convert your CM into a cafe racer; if that still applies, then using whitewall tires would ruin the cafe look entirely.
Obviously, you're THINKING, and that will pay off as you develop your ideas, but the creation of a cafe racer requires a distillation of the original motorcycle into a more elemental machine. Form follows function is the creed; in building the ultimate cafe racer, you'd remove everything that can be safely eliminated while still keeping your CM street-legal; in the real world, certain compromises are expected, such as license plate brackets... In the true cafe style, less is more. Overall, I think you're on the right track, but whitewalls are the antithesis of a cafe bike; IOW, once you install whitewalls, you'll no longer have a cafe racer; the CM will merely become just another bastardized retro bike. Whitewalls are for SHOW, while cafe is for GO; whitewalls do NOTHING to improve the motorcycle AS a motorcycle; they are merely a decoration intended for something that ISN'T a cafe racer... I can't imagine seeing whitewalls with Clubmans or clip-ons; the very idea makes the bile rise in my throat. Leave whitewalls to the "bling-bling" crowd, who have no real vision; they simply follow the multitude of other know-nothings, like the brain-dead sheeple they really are... I fully admit that whitewalls have their place, but that is not a place where you'd ever find a true cafe racer...
I hope that you'll choose to keep to the cafe racer spirit; I think you have a damn good platform for it, especially since this is your first bike. I realize that you're probably just bouncing various ideas off your DTT brethren to get their opinions, and I commend you for that. If you decide to go retro with your CM, then you should probably start looking for wire wheels, and swap your front disk brake for a TLS drum; think of a vintage Indian, or a pre-AMF Harley, if you truly want to go retro. Whatever else you do, don't stop thinking about turning that Honda CM400 into YOUR CM400; I get the impression that you're still running your ideas through the mill, developing your cafe instincts, to see what develops, and that alone tells me that this is going to be a VERY interesting build before you're finished with it...
Others might disagree with me on this, but I think that if you decide to haphazardly incorporate various elements of other styles into your CM, then you will end up with a mish-mash of components, all hung together on a frame, and nothing will look right or balanced. Cafe racers aren't drag bikes; they aren't cruisers; they aren't retro; they aren't Wingabagos; they represent a BALANCED design, a motorcycle that can do just about anything well; they go, they stop, they handle... you know the rest. They take us to a place we can't get to alone; the journey isn't always easy, but it is always worthwhile. In order to embrace the cafe way, you have to be able to realize when you stray from the true cafe path... Cafe allows for a LOT of individuality, but that being said, there are cafe racers, and then there is everything else, which all fall short of the cafe mark... For real cafe inspiration, check out some of the work being done at Lossa Engineering:
http://www.lossaengineering.com/BikesCompleted.html
http://www.lossaengineering.com/BikesInProgress.html
No whitewalls to be seen... In order to fully develop your cafe vision, you have to see not only what IS there, but also see what ISN'T there... There is always silence within the music; without the silence, you only have endless noise. Learn to listen to the silence, not just the sound; learn to see the entire motorcycle, including the stuff that isn't there... trust me, the stuff that isn't there is missing for a reason...
Good luck; I hope you'll consider this in the manner in which it is intended. As I've mentioned, it is obvious that you're THINKING, and that is something no one else can do for you...
-- kd