1984 sunbird 5.7 which prop

[dmweyrauch]

I have a 20 foot open bow deep V fiberglass boat with a 5.7 260 hp engine alpha 1 outdrive. Now the boat has a 21 pitch alum 3 blade prop on the boat. It will run 44 mph on GPS @ 4900 rpm, I was wondering I have also a spare alum prom but a 19 pitch what do you expect it will do with the rpm's and speed, Ideally waht would be the best prop, I am on a river or shallow lakes the most so I will stick with alum. Thanks for any recommendations