If I understand the OP correctly, he wants to go UP in diameter. I'm not sure he can go up but if he stays the same diameter, he should use a 12x4 or 12x5 3-bladed prop. I had a similar issue with a 43 inch Corsair. The reason the Corsair had the inverted gull wing was to have the largest diameter prop for that huge engine. I used an OS 32SX which had very high RPM. It would swing a 10x6 2-bladed prop at around 18K RPM. So I put a 10x4 3-bladed. It swung that at 18 as well. I would like to have had a bigger prop but couldn't find one.