Driving sheet-metal screws into 4130

Greetings, all.

I'm restoring a Champ, and have a question about driving screws into a
drilled 4130 tube.

If I drill the proper sized hole, the screws refuse to 'take'; they simply
give up their threads to the harder metal of the tube. If I over-size the
hole, then the screw won't hold diddley (that's a technical term).

Do I just keep driving fresh screws until one finally makes it in? I've
tried both stainless and cad-plated 'pk' types.

Another related question: These screws are going into a structural tube -
the windshield pillar, in this case. The original design from Champion
appears to have been thus. Is there a danger here, other than opening the
tube to moisture and corrosion? I am making an 'extra' hole, by the way.
And, unfortunately, I can't weld it up, since the fabric and interior are
done, and the hole is through the fabric, and right next to the headliner.

Jon B.