In the General Regulations section of my 2001 NHRA rule book it states "all materials must be 1 3/4" O.D. x .118" mild steel or .083" 4130 chrome moly tubing." The cross bar behind the seats that ties the hoop together can be "1 1/4" O.D. x .118" mild steel or .065" chrome moly tubing." This was under the roll bar section (4:10) but looks like it would be the same specs for a cage.
CM tubing must be TIG welded, no porosity, free of slag, no grinding allowed. Steel can be MIG welded.
Specs may have changed since then. You would need to verify this info. Go to the NHRA website and order a rule book.