When I converted the O-320 on my RV-9A to constant speed, the engine was not a problem -- the original builder had not installed the plug normally used for fixed pitch.
I did run into a cowling issue, however. For whatever reason, even with the (presumably) normal spacer for the metal fixed pitch prop, the constant speed prop would not fit with the existing cowling. I bought a new cowling (more on that later), but later found a 2 1/4" spacer for the constant speed prop. That made things fit with the existing cowling, the only problem being a rather large gap between spinner and cowling. HOWEVER, that large gap makes it oh so easy to take the cowling on and off. I no longer look with envy on planes with really tight clearances, rather, I feel sorry for them.
Back to the cowling -- it's brand new from Van's, latest model (includes the extra exhaust fins), and fits the -7 and -9 series aircraft. I'll sell it to you at Van's price and pay the shipping -- will save you about $200. Or if you know anybody driving past Savannah, GA...
Ed