The stock DS base casting fits inside the new mast, so the mast does not bear as designed on the little flange at the bottom of the casting. I cut an additional base from 3/8" aluminum flat stock.
Here are the parts before assembly. After reading Capt'n Pauley's article on "Rust and Corrosion 101" in the May/June issue of Small Craft Advisor, I wanted to prevent aluminum-to-stainless steel contact as much as possible. I cut a shim from .050" phenolic board to go between the new aluminum base and the stainless hinge plate.
Here is the new upper tabernacle assembly. Tef-Gel is not to be found where I live. I substituted Permatex 806332 Thread Sealant with PTFE, which I found at Auto Zone for about $8. This product is actually a pipe dope, but it is supposed to seal the threads, prevent galling and corrosion, and both lubricate and prevent loosening. That's the white stuff you see around the machine screw at the rear of the casting.
Here is the upper tabernacle assembly fitted to the mast. I'll fill the gap inside between the mast tube and the DS base casting with 5200, per advice from Capt. Penn Burke, who sold me the replacement mast. The black stuff on the mast is Poor Man's Layout Dye, aka Magic Marker.
I still need to drill and tap for the retaining screws. A drilling and tapping tutorial would be a good subject for another thread, but not tonight....