You're not wasting your time. I think you would like the performance that a modern f3a Contra Drive gives your plane.
With a Contra Drive you are going to want a much higher pitch than you would use with a single prop Drive. This is because each prop on a Contra Drive absorbs half the power absorbed by the single prop on a single prop Drive. Your 5" - 6" pitch would be equal to 10" - 11" pitch on an equivalent Contra Drive system. The lowest pitch that you can find for an f3a Contra Drive system is 18" of pitch. However, I think that this would actually work well for your plane. Anything smaller won't fly your plane fast enough to stay in the air.
Brenner ...