My shower head also comes out of the wall at about the 6'6" mark. But
the original tiles only go up to about 6', and the original drywall
above it has never had a problem with moisture. It has several coats
of oil-based enamel and that's kept it sealed well.

I don't want to run the tiles all the way up because based on the
shower's FIRST 40 plus years of service it just isn't necessary. I
live in the dry desert Southwest.....maybe that's a factor in moisture
not lingering on the drywall above the tile line.

But back to my original question......what would be the best way to
smooth out the joint between the original drywall and the new
Durock???????