As it is now with a 19p x 4000wo rpm's it's over propped.
That 140 should turn 44-4600 at wide open throttle, trimmed with normal load. To answer your question a 17p will get you in the ball park on the high end & will slow your trolling speed at 650rpms. You should also be able to tweak that carb. down some to 500-550 with no problem & achieve what your after if the 17 doesn't quite get you down to where you want to be.