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Before making a major investment in a prop ($3-800), raise the engine so that the top bolt is in the 3rd hole down from the top.

If the lower bolts are in the blind hole of the bracket, they will need to be filling in and new lower holes drilled. There is a template available.

I would run the boat with just you and a tank of gas and report back WOT rpm and speed.

You still have time this year.

I will make a forecast. Your prop is sized for a very light load. Here are two props that are recommended by owners who have the Mercury 90 2-stroke on a classic Montauk:

13-1/4" x 20" Laser II / Lightspeed
13-1/4" x 19" Turbo 1 / Stiletto Advantage

Target speed with either of these props, running light is 43mph.