I have two .53's one in a pattern plane and one in a tank the best prop i have for the pattern is a 11x7 or 11.5 x 7 and use 5% Nitro with castor/synthetic mix
The 12/6 bogged it down, it had a lot of power but was slow.. so maybe for scale flights on you p-51 will be good, I'ld use the 11.5 x7 for everyday flying.
Also make sure you have enough airflow inside the cowling to cool the engine, and finally do not run it rich!!