my opinion -- fwiw

If you have access to a portable welder, I'd probably buy a length of redi-rod and just weld short pieces of it to the pipe -- torch a hole in the guardrail at the right spot and flat washer and nut  -- you're done.

redi-rod will allow the guardrail to be bolted tight to the pipe -- won't have the thickness of the bolt head holding it out from the pipe and thus allowing the gardrail to pivot a little on the bolt.

If no portable welder, then torch a hole through the pipe and bolt through the pipe.