first of all you can buy a 13.5x 18 or 13.5 x 20 or 13.5 x 22 and even a 24 pitch in the omc raker series for this engine.
you need to raise engine up so centre of the propeller shaft is about 3 inches under the bottom of the boats transom.
forget about the cav plate, you dont need to worry about that for now. and at wide open throttle the cav plate should be nowhere near the water to get a bit of speed.
get a long tube and fit to the tell tale so you can have that up the front to watch to make sure you get good water flow from it, or better still get a water pressure guage.
The 13 x 19 alloy is rubbish compared to the 13x19 sst from omc, they also make a 21 sst and a 23 sst for your 13 splines.
also you can get a 13 3/8x 21 ballistic that has huge amount of rake.
Now dont worry about over revving as that is not going to happen, these engines can rev to 6500 all day long and not harm them.
your max power is at around 5800 to 6200, forget what it said about the 4500 to 5500 in the brochures. that is for the cruiser brigade and those who dont understand these engines at all.
forget about ordering a brand new prop, go on ebay there is thousands that will fit your motor.
a 13 x 21 omc sst can be found cheap on ebay, and the best for you would be a 20 inch or 22 inch omc raker, that will give you 13.5 x 22 pitch and go like a rocket.
there is plenty of them around and the older ones are better than the new ones made under license by solas, the newer raker is weaker.
I have 8 different props and ranging from 15 pitch to 23 pitch, and I would like to get my revs up to around 6500 with a 19 pitch.
your prop slip figures are not to bad but could be better, get the tank and battery as far back as possible to let the bow come up and get boat moving quicker.
if your boat rides level your losing tons of speed, it needs to be bow up and engine level with water if possible, less wetted area and good thrust from prop.
this is where the raker props come into play, they get the bow up and give real good top speed.
with a 22 raker youl be going pretty fast and being a vented prop your holeshot wont be too bad neither, but if you want fast pull away then a 13x19 omc sst and vent holes will be the right prop. top end wont improve much more than 2 mph, but getting there will be better.