"Your frame rails are more than likely heat treated so I personally would avoid welding anything directly to them unless you're an experienced welder. You could weld your supports to a plate and drill 4 holes in the web of the rail and bolt the plates to your frame rails."
Absolutely the best way to go if you feel you need to go all the way to the frame. I'd probably just go with tubing welded to plates attached to the roof bows. Unless you plan to tote an Abrams tank up there or sumpin? Jack