That is really cool but I bet you the ratios are different for the Japanese and Italian speedometers. I did some research on this for my scrambler project, for which I wanted to use an old style speedo/headlight combo.
An alternative way to do it would be to buy a headlight bucket like this from roc city cafe (
http://www.roccitycafe.com/store.php)
Similar to stock but deeper and you could mount a speedo in there. The stock speedo might work or you could go with a mini speedo like this from
http://www.crc2onlinecatalog.com/Index_Main_Frame.htmOne other thing I was considering for my project was to simply buy an earlier Honda headlight from a CB77 or something. The problem with that is they used an 18" front wheel as opposed to your 750's 19", so the speedometer wouldn't be accurate. The CL77 I believe used a 19" front wheel though, and that might work.
Good luck and I like your idea. I'll be posting a build thread on my scrambler when I start it so you may be able to see what I decide on there.