I am building the GP cap 232 and plan on using an OS 50 SX engine. What prop would be a good choice for this combination?

O.S. suggests a 10 x 8 or 11 x 7. For more vertical, perhaps an 11.5 x 6 might work. Play with different props to see which one gives you the performance you want. As long as the engine's turning in the mid-to-high 12,000's, you'll be OK.