If you must do this, and I agree with Ronald and Larry - It's not worth it.

1) You must separate the stainless from the aluminum mast with sheet plastic to prevent large patches of electrolysis damage. Paint is not enough insulation.

2) It is less labor and less electroysis issues to use monel pop rivets than to tap.

3) I you do tap be sure to use Duralac or Tefgel on the threads so you can get it apart in the future. I prefer Duralac to Tefgel for screws because Tefgel is like a grease and screws may loosen over time, especially in high vibration areas. Duralac hardens somewhat and I think the screws stay in place better.