Sterndrive
 

If I was to replace my 3.7 with the 3.0 would I have to replace the upper part of my sterndrive so the gear ratio would be right or could I just use the same one without modification?

you have a 1.84 ratio now, the 3.0L uses a 1.98 ration. Depending on which prop you are running, you could probably step down in pitch enough to make up the difference

How much should I step down from? Right now I have a 21 degree pitch

I'm surprised you could spin a 21, i would have guessed you would have had a 17 or 19 with the 3.7, we used to run a 21 on a 17 stingray. that being said, i would start with a 17 and see how many rpms you will turn at wot, make sure your tachometer is accurate, you want to see at least 4400 at wot. You may have to go down to a 15

Cool. Thanks bud. I will give that a try and see how it goes.