ideally someone with the same engine on a 6.5 boat will chime in their own findings, in the mean time-
as a GENERAL guide you change 200 rpm with every 1" of pitch change. looks like it is a 17" prop you are looking for then, maybe also drop the dia down to a 15" perhaps worth looking at.
i try to prop for 5650-5700rpm (others may vary ofcourse) with my normal load and a full tank of juice, as fuel goes down it will get into the butter zone....in theory!
in case no one with a similar rig responds, more info can be seen here on the zuke site-
look at similar weight boats here and props they spin- http://www.suzukimarine.com/Product%...012/DF150.aspx
take for example- http://www.suzukimarine.com/Product%...0220FX%20DF150
it is a bit heavier, deep ish V and swings a 15x21 up in the 5800rpm range with 140 ish litres of fuel on board...interesting IF accurate!
assuming (dangerous) the info on the zuke site is accurate that would suggest perhaps your motor isn't making all its power. could be fuel filters maybe or perhaps there is water in the hull or outboard too low etc?
it may be wise getting a print out of the motor diags before doing anything else, it will tell you how many mins have been spent at what revs. assuming the original owner never changed the prop that will give you an idea where to go.
without question though, i would get the RPM up one way or another.
someone hopefully along with similar setup soon with more bespoke info, but the above should hopefully get you an idea on prop sizes.