I'd use DOM (drawn over mandrel) tubing, not schedule 40. When I was in high school a friend built a "roll cage" for his topless home built dune buggy out of pipe (didn't know how to weld so he juts bought 90's, street 90's an T fittings) and got really lucky in a roll over - he got thrown clear and the cage collapsed. Schedule 40 is fine for its' intended use - transfer of gas or liquid from point A to point B - it doesn't have the structural integrity of a good piece of DOM chrome moly tubing ...
Just the opinion of an old fool - but I've survived a LOT by listening to myself ...
Cheers
Steve