RE: 3 OR 4 OR 5 blade prop
What RPM are you getting with that 13 pitch....Remember the higher the pitch the lower the RPM's...For every pitch you go up you lose around 400 rpm's and if you go down in pitch you gain about 400 rpm's.I think youwould want a 11 or 10 pitch,we put 9 pitch on our big foots 40-60 hp.
see what rpm's you should be running with that motor.....and as far as diamiter goes,as long as you dont hit the anoid or casing your good there.
If I remember I will ask a tech at work tomorrow and see what they say...