How much does your P-51 weigh? I'm flying mine with a Zenoah G62 and 2 blade Mejzlik 22x10, my P-51 weighs in at 29 lbs. If you want turn a 4 blade prop directly then maybe 3W75 is a good choice. I have 3W75 in another plane with a 24x10 mejzlik and it has signficantly more power than the G62. Not sure how the 3W75 will do with a 4 blade.