A1011 HRPO isn't stainless steel.
The ATSM A1011 specification covers hot-rolled, carbon, structural, high-strength low-alloy, high-strength low-alloy with improved formability, and ultra-high strength steel sheet and strip, in coils and cut lengths.
HRPO stands for Hot Rolled, Pickled, and Oiled. It's process that removes surface corrosion after the hot rolling process. After the steel is hot rolled in to shape it is washed in inorganic acid to remove surface corrosion (inherent to the hot roll process) and then oiled to preserve protect the surface.
It's not stainless and it's not commercial steel. It's very high grade structural steel that has been treated to prevent corrosion.
You could weld stainless 304 primaries to this with 309 wire, argon, and a mig welder. Keep in mind when doing it though that it's really easy to blow through thin tubing with a mig. I tend to crank up the heat and wire speed and weld a series of spots all the way around the tube. Let it cool between spots.
I get good *********** and a flat, wide weld.
This is what I'm planning on doing for my mustang.
-Zach'sV6