Check out the Mercruiser site at
http://www.mercuryracing.com/propellers/maximus.php this will tell you the difference between the props. The Maximus props are very good for your application. Also the Hydromotive props work good you can check them out at
http://www.hydromotive.com/ both these sites offer tech advise on the difference between the 4 and 5 blade props. The five blade are generally more money but perform better at all rpm ranges. The 29p to 30p sounds correct as the 38 with 502's 425 HP turn 28p props. There a great boat dealership in your area called The Boat Connection the owner is a world champion race boat driver and actually owns a PowerQuest boat he is very knowledgeable about props the number is 561-686-0002 ask for Howard.