Hi daniel,
I have been useing 1.75 x .062 Cro Mo tubing and 1.625 x .062 will fit inside with a little trimming for adjustable booms etc. I use oxy-acetelene to braze the parts togethter and have not had a weld fail in 10years. Just remember that keep the weld areas clean clean clean and it will go together real nice. Here's my bent building page
www.bobmccutcheon.com. Keep us posted on your progress.
Bob McCutcheon