I run a Mod OS and the X640 didnt go well with it so I dont think you will get the RPM needed from the OS with a stock muffler/ stock setup. Use a X437/3 ITs the best prop Ive found so far mod or stock, Ive tried about 20 different props, and the x437/3 is #1 and prather 215 is #2
The setup tips from WB is a good way to find out where you are. the O.S. does not like to run deep in the water, however 1/4 or 5/16 may be to high and you will get cavitation and not enough thrust, Alot depends on the boat, I would recommend running o degrees on pitch and start with the center of the prop shaft even with the bottom most edge of the tunnel. then move up a 1/16 at a time until It picks up "bad habits". when you play with the trim angle you may have to adjust the height again as the center line of the prop shaft will rise up or lower down depending on which way you go. Go in very small increments on both height and trim angle. I run 2 degrees positive on trim and centerline is 1/8 above the bottom of the sponson, with no weight (kinda rare).......... (this will be boat specific as to how much weight you should add if any). With my mod K&B /scratch built tunnel I have to add 3 oz. to the front.
Hope this helps ~ James