Selecting correct transmission ratio

[DAYTRIP]

I am a (recent) former bertram owner now rebuilding a inboard skiboat. I have no engine or tranny so I am at ground zero. I have an opportunity for a zf 45 that is a 1.5:1. These things typically have a 1.25:1 in them so I am not sure if beyond a prop change I am going the wrong way. Are there any rules of thumb for selecting what transmission ratio should be used on an inboard application? Speed of these boat is typically 45mph WOT.

Thanks for your help,
Jim