As someone who recently bought an RV-6 with O-320 and fixed pitch wood prop, I'll throw in my 2 points for opting for the O-360.
Taking off with two aboard, full of fuel, on a hot Texas day with the density altitude up there... I sure wished I had an extra 20 horses!
(Anybody ever put a nitrous oxide injection system on an O-320 for takeoff boost?

)
The O-320 and wood prop combo can make some aft CG issues on a -6, especially with the baggage compartment loaded up and as the fuel tanks get down towards empty on long x-c flights (e.g. going to Oshkosh). The -6A has the benefit of extra weight from the nose gear to help out here, but the extra weight up front of the 360 engine is even better in this regard. Nose gear plus 360 engine up front means you can carry more stuff in the baggage compartment without worrying about getting too tailheavy.