Got pics of your cage setup?
The traditional method was just to have tabs welded to the down bars and where they crossed paths with the body, they were bolted on.
This was usually only at the front main downtube, because the bar maker could add the tabs where they could be bolted to the existing threaded holes for the bulkhead. But the same thing could apply to the rear where you would just drill holes through the body and use a nut, or better yet a nut plate on the backside.
Where this gets tricky is if you have the side panels installed. In which case you'd want to mark, drill then add nut-serts or something similar so you don't have to access the backside of the panel.
The straps/tabs were usually about 1/2" wide, 3 or 4 inches long, and about 1/4" thick. Welded to the cage bars and bolted to the body.
I've seen a lot of bars built in recent years that are super tight to the body side. In these cases you could probably use welded tabs to the bar, rather than a full strap.
On the other hand, for simplicity's sake you could probably just make a clamp that surrounds the bar and bolts to the body. Just seems like stronger is better in this case.
Got pics of your setup?
Paul