I went with the 61, hoping that it can pull a 23" prop that will pull the big Spitfire around a bit better. Besides a bit more displacement the 61 has a 10.5:1 compression ratio compared to 7.6:1 for the 55RA (If you believe the specs).
Got my Best Pilots Sailor Malan pilot in the mail. At 1:5 scale he's a bit small, but the cockpit is quite shallow compared to the real thing. The fit looks decent, and his body fits well sitting on the shallow floor. I was also able to coax the instrument panel out. It's glued in with silicone, so it takes some patience and work to get it out. Since I'm scaling this thing out, I might as well put some extra time in the cockpit. Here are a couple photos showing Sailor during some fitting tests.