Small boat like that with 4.3 V6 should easily make 45 mph. Seems to me that the drive ratio is not ideal with this combi. You can try to compensate with smaller pitch prop but you will not get 'the most' out of it. If possible try to swap to 1.8 drive and the try a +/-14" dia 23 pitch propeller.
More diameter will make the RPM's go down so that would not make any sense. The party barge seems like a boat that will be heavy loaded. I would suggest a 4 blade prop with smaller dia and pitch to reach the max rated WOT with a low load.
To really help you there should be more information about your RPM at WOT and the specs of the current prop. If you want an aluminum prop I would focus on the Quicksilver Nemesis (also known as Mercury Spitfire). It is not the fastest prop but will give you good holeshot. Keep testing different...
