Well that's great news so what could I do other than changing to another engine to get it up on plain if it turns out it doesn't maybe a a larger pitch prop?
I used to own a very similar boat. It came with a 3-cyl 60 HP Evinrude. I later upgraded to a 3-cyl 70 HP Evinrude. Boy I loved that boat and wish I still had it now. It wasn't the fastest on the lake but it ran faster than I wanted or needed to go. We could run with 4 adults and 2 children without a problem at all.
That 70% number is merely an opinion and it has no basis in fact. Prop that 70 so that at WOT you just hit the maximum rpm spec. If you don't know what that number is, google it. If planing is a problem, you need to run a smaller pitch, not a bigger one.
Personally, I prefer to run a cupped SS prop to achieve the best performance. Tell us what your max rpm is, what prop you have on now, and we can tell you what new prop to buy. If you don't have a tach to get the data, buy one. That'll be a lot cheaper than a new motor.
Having said all this, if your 70 is worn out, poorly maintained with bad compression, etc, etc., then optimizing it is a lost cause.