Attaching hardware to spreader.

[sailak]

I would like to rig flag halyards to the lower spreaders.

Can I drill holes in the underside of the spreaders?

If so, do I tap and use screws or would pull rivets be a better solution?

Aluminum spreaders.

Thanks

[knothead]

Either method works fine. If you are using shackles with the blocks, it might be easier to use pop rivets. Use stainless rivets.
If you are attaching the blocks directly to the eye straps, you might want to use machine screws in order to allow you to remove the blocks when they wear out.
Use something on the fasteners like Lanacote or Tuffgel to ward off corrosion.