prop question

[BobNewbie]

I just purchased a used boat. The motor has very good compression even though it is an older motor. I ran the boat and it seems the top speed is around 25 MPH. at 4500 RPM. I would like to increase this speed slightly.
Earlier here is the boat specs:

Boat is homemade from a mold of a Boston Whaler hull. so hull is shaped like a Whaler.
Hull Material is fiberglass
Length 17 foot

Motor is a 1984 Envinrude 70 HP with a 3 blade 13.25 X 17 prop.

My qeustoin is what size prop pitch should I try to go to achieve higher top end.

[meepbobeep]

But at the same time when you order a prop, call Michigan Wheel or whatever company and ask what size prop you can use and what RPM and speed to expect.