Me, I'd keep the black ones in my "treasure chest" of parts even if I put white ones on. I have one black one, NOS and it's staying with me. I hope you still have the plastic lensed tach and speedo, a good set went for about $800 on e-bay not long ago. I bought a 1969 in August of '69 but I can't remember if it was a sandcast which are the real rare ones. Once Honda figured out the had an absolute winner on their hands they went to die cast. I just bought a die cast motor that appears to be built in late '69 early '70.
Check your engine and frame numbers for build dates, you may have a 1970. And, keep all the original stuff, you'll drive the guys that know what you have nuts. Check the build date on the steering head, that will tell the story. Also, another good book is Honda CB750 by Mark Haycock.
Sorry for the rambling answer - it's saturday nite and a bit of red wine has been had - but I think you may have a real interesting bike there.
Cheers,
Cliff.