Hi Hank! I got your PM (didn't see it until just now) and yes those are the star nuts. I bought the same ones from McMaster. Once you get a new bolt screwed in to a star nut, you sorta tap it in with a hammer. You basically only get one shot at it, so try and keep it straight.
But here's the deal. If it goes in really easily, the inside of the frame is rusted and the star nut won't hold. I suppose there are slightly larger star nuts?
Good luck on the cross brace idea, it's likely needed. I'd be tempted to drill a hole straight through the frame and shove a bolt in to secure at both ends -- possibly using a ferule to lessen the chance of the hole becoming oblong.
I wanna say I've used regular black BBQ spray paint on one of the old gasser frames in spots.