Normal cap head stainless bolts do interfere with the speedo drive...all you have to do is file about 2-3mm off the drive where it interferes and - hey presto, problem solved! I have done this on my old 748 and my new(er) 916SPS and have now on-going problems.
Stupid question, but why don't you use the standard ducati ones? They have a low head, but unlike the button headed allen screws, have a nice, deep hex. Not that expensive, even from Duc.