Master Angler prop

[BH Escoe]

After quite a bit of research on this site I would like some advice on a prop. I have a 1977 Master Angler with a 1979 Mariner V175. I know the motor is too heavy but don't have the cash for a re-power at this time so I am forced to make the best of what I have. The boat came with a 21p quicksilver aluminum prop and I am quite sure it is over propped. I'm not sure if I should try a 19 or 17 to start off in my prop quest. I don't have any numbers as far as rpm but it does cruise around 25 mph and the lowest speed I've been able to stay on plane is 18-20 mph. Any help on a starting point would be greatly appreciated. Thanks in advance for your help.