I haven't had the experience of v-drives just yet. My opinion goes only toward the motor side. I am assuming you already looked at the obvious such as the carbs opening fully and evenly and your tune-up is good. But I am a Chrysler fanatic.
Two things that come to the top of my head are, valve springs and the cylinder head. I think 6000 RPM's are pretty good for a stock unported 915's. They are one of the two best stock heads. Are the valves stock also? Tiny by todays standards 2.08/1.74. You could benefit from a bowl port and 2.14/1.81 valves. I have 915's with the larger valves and a bowl port, and an 850 in a jet. My 440 will rev easily to 6500. At this RPM you better be careful with oil. You need at least
a 1/2 pickup, or better yet, an external pickup like the Milodon setup and a huge pan.
I am guessing your valve springs are more likely the problem. With .600 lift it doesn't
take more than a hard season of boating to kill a set of springs.
Brian
[This message has been edited by wsuwrhr (edited July 09, 2002).]