Does anyone know what mods are needed for these lower unit swaps?

[JohnnyMat]

Trying to put the lower unit from a 1978 Johnson 35HP long shaft on a 1976 Evinrude 35HP long shaft.

Engine models:
1978 Johnson: 35EL78R
1976 Evinrude: 35653G

I don't have the lower unit from the '76 because it was stolen. I only have the '78 lower unit. Lower unit has the drive shaft, shift rod, water tube, and prop.

Looking for help to see if this swap is possible and which parts I will need to do the mod.
Any help would be greatly appreciated.

 

[Bosunsmate]

They look pretty similar beasts. Have you tried installing it? If it fits make sure the impeller is not worn out

 

[JohnnyMat]

They appear similar but I haven't tried installing yet. I won't be up at the boat for another few weeks. I am trying to have my ducks in a row before I even bother trying to install.
There are a few minor differences on the internal components of the gear case. However they don't appear to be anything I would need to be concerned with if I am simply doing a bolt on. I'm not a professional though. The main thing I see, that "might" affect connecting the '78 gear case to the '76 power head, will be the different O-rings on the impeller spacer tube between the two years. The two years have different impeller housings/water pumps, so I don't know if there is a nominal height difference between them that would affect the gear case and power head sealing to one another properly . Hope that makes sense.

Here's a pic of the two lower unit schematics side by side. As you can see the 76 shows a few more o-rings needed. They also have different water pump housings but use the same impeller spacer. Not sure if I should go with both '78 O-Rings or go with the '76 O-rings on just the top of the impeller spacer. There may also be a height difference between the two because of the spacer plates and gaskets below the water pump assembly.

I will obviouslyl need to use the '78 water pump assembly if I use the '78 drive shaft (already on gear case). Otherwise I would need to purchase a '76 drive shaft and '76 water pump assembly. My concern with using the '76 stuff is the difference in the drive shaft seals/spacers/needle bearings inside the gear case. (circled in pic). Don't want to open a can of worms and start toying with the internals.