The -C drive has very different internals but I will look at the feasibility of reworking the -B housing to receive the -C guts.  I think it can be done.   Yes, the -C bolts right up to the same adapter plate.  I think you are right about the 2.85 becoming the preferred ratio, but only if you can handle the longer prop.   You nose draggers have the advantage here.

The more I fly it the better I like this setup.   The higher rpm was very disconcerting at first but I acclimated rapidly.   And now that I have  digested the fact that the actual rpm difference at normally used throttle settings is only about 5%, I absolutely love it. Another good sign is that the manifold pressure is now more than 5% lower at any given airspeed that I've tested so far.  Even if the wear rate is up 5% or so it would be a non issue. 

One more plus for the 2.85 is something I hesitate to mention.  It's kind of like the "engine making oil so I have to drain some out" thing, kind of unbelievable.   It makes sense that there would be less prop noise but I'm also getting less engine noise. 

I was getting tired of the increased noise with the Hushpower II muffler and was almost ready to put the Spintech back on even though it costs at least 5 - 6 mph in drag.  But with the -C drive things have quieted down substantially.  I think part of the credit for this goes to the difference in RPM moving the vibrations away from the resonance point of the sheet metal panels in my RV-4 but even observers on the ground have mentioned that the engine sounds quieter.

I better shut-up now, this is starting to sound too good to be true.

Tracy