Technical LaSalle Ford Closed Drive Conversion

I'm currently working on converting a side shift laSalle 3 speed to a ford closed drive as described in an old thread here:

https://www.jalopyjournal.com/forum...onversion-courtesy-of-bruce-lancaster.408427/

Now, my question is when you add the ford bearing retainer to the lasalle trans you lose the main shaft seal on the tailshaft, wouldn't that cause a trans leak issue? Maybe a sealed main shaft bearing is needed at the back of trans? Maybe I'm over thinking this.

Thanks