If I were going to put a tang on I would take electrical tape and tape the area of the mast that the tang sits on. You can find electrical tape that's 3-4"s wide. This insulates the stainless from the aluminum.

Take a razor and poke it through the tape where the holes in the mast are.

Use Tefgel to coat the holes in the mast.

Place the tang and using a pneumatic riveter set the rivets. Do not use a cheap hand riveter.

Carefully trim the excess tape. Don't cut into the aluminum.

Using mineral spirits wipe off any excess tefgel. Then you can use something like shoe goo, or aqua seal to dab over the mandrel holes.

Use the right length rivet.