If you can find, use Monel rivits. These will not cause the corrosion
issues SST will. If you do use SST, then coat in lanacote before placing them in the hole. If the step is SST as well, then you need to use a urathane adhesive sealant
like 3M5200 or Sikaflex or any similar, as a bedding compound. A small amount will squeez out. Just wipe clean with some turps on a rag.
If you did go for a thread, you always want to use a corse thread like UNC etc. on any threads under 8mm(5/16). From 8mm up you can use the finer metric threads. Coat threads in lanacote. For speeding up tapping the thread, you can use a battery
drill on a slow speed and a good starting tapping. Just spin the tap all the way through. It's quick and works well.
Be very cautious of using Aluminium rivets. They are soft and can shear without you ever knowing they have. Monel is strong and corrosion
Most mast sections are similar in thickness all the way around. But watch for twin section extrusions. These will have a seam down the sides and the mast is twice the thickness along that seem. A rivet slightly longer than needed will still take up.