I would use 4130, how do you plan to hold the pieces square while everything is fitted and brazed? I ask because we build airplane fuselages out of 4130 tubing, and fitting all the tubes together and holding them in place is an involved process. You might want to find someone who knows how to braze and let them teach you, I would hate to see something on your frame break and you get hurt. I'm not trying to discourage you, but make sure for your own sake you know what you are doing,.