Changing gear sets. 1971 mercury 500

[Jdgdtt]

Looking for help. Thanks in advance.

I messed up the gears in my lower unit. I bought another one to rebuild but it turns out the lower in my motor does not have the same driver shaft as the one I bought. Mine has an extension so I can’t make it work as is. i want to swap the gears in to my lower so I can use my drive shaft. The current gear ratio is 2:1 and the new is 1.64:1. Do I need to match a prop to keep from damaging the motor?

 

[Faztbullet]

That lower is for a 3 cylinder motor and you will need to reshim the entire unit if you swap gears if they will swap. Do they both use the cone type roller bearing on DS?

 

[Jdgdtt]

Bearings all appear to be the same, the forward gears both have cones on them and they are the same bearing. My motor has an extension on to too. This is all getting scary. What a mess. I measured all the gears and the are the same shaft and bearings. Im worried about where the pinion gear will match up on in a different drive shaft and getting the right prop for the gear ratio.