A few more tips from post on the main page.

If optimal performance is one of your goals (as I believe it should be), then make sure you have the correct shaft length of engine, relative to the boat transom. The cavitation plate (the plate right above the prop) must be level with the lowest point of the transom. That way, you will get top performance and reduced chances of over/under rpms on your engine; thereby increasing engine life and optimizing your top end speed.

Some marinas try to stick any engine they have in stock on your boat if they have excess inventory, so it's best if you know what is needed up front.