The first thing you should buy after getting any boat is a copy of the engines factory SERVICE MANUAL! Those old carb'd motors are basically very simple, so you can do a lot of your own maintenance, and the linkage adjustments are clearly illustrated in the manual. Most engines that are overpropped (too much pitch) are typically SLOW to get on plane, so you may be right that the motor isn't developing full power, for whatever reason. 4200 rpm is way too low for that motor. Just pull the air baffle off the front of the carbs and with the ignition off, advance the throttle to max and then look at the carb throttle plates to make sure they're wide open. If not, there is a knurled knob on the throttle cable under the cowling that is adjusted to put the throttle butterflies on the wide open stop. There is also an adjustment that determines where the throttle is when you hit full spark advance. On my old V-4, there was a mark on the throttle cam that lined up with a roller on the throttle linkage when the spark advance linkage hit it's stop. This is all described in the manual.
If the linkage is ok, the next step is check basic stuff like ignition timing and compression to make sure the motor is mechanically healthy with no bad rings or scored cylinders. If compression variation is within 10% on all cylinders, the next thing I'd check is to make sure you don't have a clogged high speed jet, as this can quickly kill a motor by leaning out a cylinder and overheating/scoring a piston and or cylinder wall! Easiest way to do this is have someone else run the boat at WOT with the cover and air baffle removed while you look down the carb barrels with a good flashlight. You should see a big spray of fuel in all 6 barrels. If one is spraying less fuel than the others, IMMEDIATELY come off plane and head back to the dock at idle speed! The low flowing carb will require a teardown, cleaning and rebuild. If one carb is full of gunk and varnish from old fuel, it's probably worth cleaning all of them and replacing all the fuel lines on the motor. I've seen old fuel lines with a film of varnish in them that can break loose and plug up a carb!
Your owners manual or the service manual will specify the WOT operating range, but typically the OPTIMUM rpm is right in the middle, so if it specifies a range of 5000-6000 rpm, you'd like to see 5500-5600 @ WOT with the max load you normally carry. Of course this assumes the tach is accurate, and many analog tachs are notoriously inaccurate even when new, so after making sure the engine is developing full power, have a dealer compare your tach reading with a known good tach to make sure it's accurate. Once you know the tach is ok, the next thing to check before changing props is to make sure engine mounting height is correct. When boat is up on plane, you should be able to clearly see the top of the anti ventilation plate above green water; this typically requires the AV plate to be about 1 inch or more above the bottom of boat.
A 150 is PLENTY of power for an 18, so unless the boat is full of waterlogged foam, I'd think you could spin at least a 17 or maybe a 19" prop.
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