For a fullsize Chevy, the 1.75" .120 wall is probably the right choice.....DOM if you can make the stretch with the $$$ (It's your life and your family, so I'd think long and hard about it)
I've used 2" .120 DOM in the past, but like you said it's a little large and doesn't always tuck in tight to the corners and edges of the roofline, so headroom is often an issue.
Like most things....the quality of the design and it's construction will be more important than the specific tubing material you choose. There are plenty of photos of well-designed cages with proper corner gussets and triangulation, and I'd definitely leverage what's already been done if I were you.
I've used 2" .120 DOM in the past, but like you said it's a little large and doesn't always tuck in tight to the corners and edges of the roofline, so headroom is often an issue.
Like most things....the quality of the design and it's construction will be more important than the specific tubing material you choose. There are plenty of photos of well-designed cages with proper corner gussets and triangulation, and I'd definitely leverage what's already been done if I were you.
